Here are my suggestions. I am less confident about the ones marked with question marks.

GENERAL:

(1) replace "<!-" and "->" with "<!--" and "-->

(2) change "Õ" to "'" in declarations

(3) For consistency, capitalize all initials that refer to individuals.

E.g. (I've tried to list them all in case you want to make substitutions for the whole chapter):

resp="mp" -> resp="MP"
resp="etd" -> resp="ETD"
hand="mb" -> hand="MB"
hand="dhl" -> hand="DHL"
id="heol" -> id="HEOL"
hand="heol" -> hand="HEOL"
hand="ss" -> hand="SS"
resp="das" -> resp="DAS"
resp="fb" -> resp="FB"
hand="wj" -> hand="WJ"

(4) I appeal again for bands of cert rather than percentages:

"A" (beyond reasonable doubt): 95% <= conf
"B" (probably correct): 50% <= conf < 95%
"C" (doubtful): 0% <= conf < 50%

where "conf" is the editor's level of confidence.

(5) The pizza maker goes awry when the textcrit topping is added.

(6) On a completely unrelated matter, I would like to have TEI Lite allow a note in a docAuthor so that footnotes concerning authors can be encoded more easily.

SPECIFIC:

p. 413, para. 7:

check font and size of "TEI.transcr"

p. 413-4, document type declaration:

I cut and pasted the given declaration

<!DOCTYPE tei.2 PUBLIC "-//TEI P3//DTD Main Document Type//EN"
"tei2.dtd" [
<!ENTITY % TEI.prose ÕINCLUDEÕ >
<!ENTITY % TEI.transcr ÕINCLUDEÕ >
<!ENTITY % TEI.textcrit ÕINCLUDEÕ >
]>

into an XML document. A few things needed to be changed before the document would parse:

<!DOCTYPE TEI.2 PUBLIC "-//TEI P3//DTD Main Document Type//EN"
"tei2.dtd" [
<!ENTITY % TEI.XML "INCLUDE">
<!ENTITY % TEI.prose "INCLUDE" >
<!ENTITY % TEI.transcr "INCLUDE" >
<!ENTITY % TEI.textcrit "INCLUDE" >
]>

The required changes were "tei.2" -> "TEI.2", 'Õ' -> '"', and inclusion of the TEI.XML parameter entity. The need for these changes may well be covered elsewhere in TEI P4.

18.1.1, para. 1:

change "described in section 6.5, Simple Editorial Changes" to "described in chapter 6". (<abbr>, <expan>, and <hi> are not covered in 6.5.)

--hand attribute of <add> and <del>:

change "the hand of the agent which made the deletion" to "the hand of the agent who made the deletion"? (Same wording used in 6.5 as well.)

--last para.:

change "Examples of more complex application in scholarly transcriptions of these core elements are given" to "Examples of more complex applications of these core elements in scholarly transcriptions are given".

18.1.2, para. 1:

change "may be represented by significant marks" to "may be represented by specific marks"?

--para. 2:

(1) I have long wondered what to call the superlinear line that represents 'n' (also used for 'nu' in Greek MSS). 'Macron' does not seem appropriate for a consonant. It could be called a vinculum (Unicode character 0305), except that a vinculum is meant to group characters beneath it. There is a technical term for the Greek version, which is too long to be useful (nu ephelokustikon, or something like that). Perhaps 'overbar' is better?

(2) change "<expan>per</>, <expan>re</>, <expan>n</>" to "<expan>per</expan>, <expan>re</expan>, <expan>n</expan>".

--paras 3 and 4:

(1) change "<expan abbr="&p-underbar">per</expan>sone" to "<expan abbr="&p-underbar;">per</expan>sone" (missing ";").

(2) change "<abbr expan="per">&p-underbar</abbr>sone" to "<abbr expan="per">&p-underbar;</abbr>sone"  (missing ";").

--p. 417, para. beginning "Observe that ...":

(1) change "element, (i.e., the expansion)" to "element (i.e., the expansion)," (move the comma).

(2) change "In the case of the use of these attributes with the abbr, the cert and resp attributes are defined as indicating respectively confidence in the expansion held in the expan attribute and the responsibility for suggesting this expansion" to something like "When used with the abbr element, the cert attribute indicates the level of confidence in the suggested expansion and the resp attribute identifies the one who supplied it".

(3) change "If it is desired to express aspects of certainty and responsibility for some other aspect of the use of these elements, then the mechanisms discussed in chapter 17, Certainty and Responsibility should be used" to "The mechanisms discussed in chapter 17, Certainty and Responsibility, should be used to express certainty and responsibility for other kinds of uncertainty associated with abbreviations and expansions"?

18.1.3, para. 1:

(1) "Fredson Bowers (Cambridge: Harvard University Press, 1977)": for consistency, place bibliographic details in the footnotes.

(2) change "inital" to "initial".

--p. 418, para. beginning "In this example":

add a comma after "Memoirs of My Dead Life".

--footnote 56:

change "H.Cahoon" to "H. Cahoon" (space missing).

--footnote 57:

change "pl" to "pl."

--para. beginning "Further information":

change "held in the attribute value of the sic elements" to "held in the attribute values of the sic elements" (there are multiple values)

p. 419, para. beginning "The cert and resp attributes":

(1) change "element, (i.e., the correction)" to "element (i.e., the correction)," (move the comma).

(2) change "In the case of the use of these attributes with the sic element, the cert and resp attributes are defined as indicating respectively confidence in the conjecture held in the corr attribute and the responsibility for suggesting this conjecture" to something like "When used with the sic element, the cert attribute indicates the level of confidence in the conjecture and the resp attribute identifies the one who supplied it".

(3) change "If it is desired to express aspects of certainty and responsibility for some other aspect of the use of these elements, then the mechanisms discussed in chapter 17, Certainty and Responsibility should be used" to "The mechanisms discussed in chapter 17, Certainty and Responsibility, should be used to express certainty and responsibility for other kinds of uncertainty associated with corrections and conjectures"?

p. 420, para. beginning "Similarly":

"Eloi, Eloi, lama sabachthani (Pierpont Morgan MA 1892, Klinkenborg 129)": for consistency, place bibliographic details in the footnotes.

--para. beginning "If deletions":

change "when they are classified by the manner in which they were effected, or by their appearance, however, this will lead to a certain arbitrariness in deciding whether to use the type or the rend attribute to hold the information" to "however, when they are classified by the manner in which they were effected or by their appearance, there will be a certain arbitrariness in deciding whether to use the type or rend attribute to hold the information"?

--para. beginning "Further characteristics":

change "Further characteristics of the addition and deletion, e.g. the date, or ink, may be needed for detailed transcription of manuscripts. Such characteristics may conveniently be recorded as attributes of the add or del element." to "A manuscript transcription may need to include further details of additions and deletions, such as date and ink. These may conveniently be recorded as attributes of the relevant elements (i.e., add, del, addSpan and delSpan)".

--para. beginning "The add and del elements":

change "describing typically brief additions and deletions in the text being transcribed" to "describing brief additions and deletions".

p. 421, para. 1:

(1) "Landsb—kasafn (Reykjav’k, 1562 quarto)": for consistency, place bibliographic details in the footnotes.

(2) add new line "<!-- hand declaration placed within handList within profileDesc of header -->" before "<hand id="HEOL" n="Helgi &Oacute;lafsson"/>"? 

--para. beginning "The text deleted":

(1) delete comma after "The text deleted must be at least partially legible".

(2) change "the gap element should be used to signal that the text was not (because it could not be) transcribed" to "the gap element should be used to signal that the text was not transcribed because it could not be read".

(3) delete from the last sentence "and section 18.2.3, Damage, Illegibility, and Supplied Text"? (The preceding sentence has already alerted the reader to this section.)

p.423, para. beginning "Similarly, the varSeq attribute":

change "The volume of markup may be reduced in SGML by markup minimization" to "The volume of markup may be reduced"?

p. 425, para. beginning "Where text is not transcribed,":

change "where text not present in the source is supplied (whether conjecturally or from other witnesses) to fill an apparent gap in the text" to "where text is supplied (whether conjecturally or from other witnesses) to fill an apparent gap"? (In the case of faded or mutilated writing, the text is still present, even if illegible.)

p. 426, para. beginning "This section describes methods":

change "Methods for recording page breaks, column breaks, and line breaks in the source are described in section 6.6, Simple Links and Cross References References" to "Methods for recording page breaks, column breaks, and line breaks in the source are described in section 6.9.3, Milestone Tags".

--para. beginning "<hand>":

change "mainLang (main language) indicates dominant language of hand" to "lang indicates dominant language of hand".

p. 427, para. beginning "In this example":

(1) change "In this example first the document hands are declared in the header" to "In this example, the document hands are first declared in the header".

(2) delete two "<-" and "->" (sic) pairs within the example (unless some comments need to be inserted).

--para. beginning "Then the change of hand":

delete "<!- ... ->" (sic; an empty comment serves no purpose).

p. 428, para. beginning "These elements are declared as follows":

change "mainLang CDATA #IMPLIED" to "lang CDATA #IMPLIED".

--para. beginning "As in this example, hand":

(1) change "here, addition but also deletion, correction, annotation, underlining, etc." to "here addition, but also deletion, correction, annotation, underlining, etc."

(2) change "The scribal or authorial responsibility for this marking may be inferred from the value of the hand attribute. The value of the hand attribute should be one of the hand identifiers declared in the document header" to "The scribal or authorial responsibility for this marking may be inferred from the value of the hand attribute, which should be one of the hand identifiers declared in the document header".

--para. beginning "As in this example, the":

change "As in this example, the resp on a particular element" to "As in the same example, the resp attribute on a particular element".

p. 429, para. beginning "The fields of reference":

change "Use of the resp and cert in interchange documents in ways not here defined may lead to unpredictable results" to "Use of the resp and cert attributes in ways not here defined may lead to unpredictable results"? (I can't think of a good way to include "in interchange documents".)

--para. beginning "Exactly the same":

delete the ellipsis in "<!- ... certainty and responsibility elements may be elsewhere ->".

--para. beginning "The choice of which":

delete "Where many statements of responsibility and certainty are made which cannot be so accommodated, it may be economical to use the respons and certainty elements throughout"? (Nothing much is gained by including this sentence. If the resp and cert attributes aren't enough then you are compelled to use respons and certainty elements.)

--para. beginning "The gap and supplied elements":

change "In many cases, however, despite damage or illegibility, the text may yet be read with reasonable confidence" to "In many cases, however, despite damage or illegibility, the text may yet be reconstructed with reasonable confidence". (Didn't he just say the text was unreadable?)

p. 430, first paragraph:

(1) "extent": change "inches" to "millimetres"?

(2) change "so that the tag must then be empty" to "so that the tag must be empty".

p. 430, <unclear>:

change "resp indicates the individual responsible for the transcription of the letter, word or passage contained with the unclear element" to "resp indicates the individual responsible for the transcription of the letter, word or passage
contained within the unclear element".

--para. beginning "The following examples":

change "V&ohook;lusp‡" to remove the entity reference.

--para. beginning "However, in fact the damage":

(1) change "However, in fact the damage" to "In fact the damage".

(2) The example does not parse as it stands--div elements don't take bare damage elements. I leave the solution up to you. 

p. 431, para. beginning "Contrast the use of gap":

change "&Thorn;ar" to "&thorn;ar".

p. 432, first para., point 5:

change "in transcription, surround the whole area with the del element (for deletion; or the delSpan element where it crosses a structural boundary); or the damage element (for damage)" to "in the transcription, surround the whole area with the del element (for deletion, or the delSpan element where it crosses a structural boundary) or the damage element (for damage)" (add "the", change ";" to "," and delete ";").

--second para.:

check font size of <add id=A1>, <del id=D1>, etc., where they occur in the text.

p. 433, <space>:

extent: change "inches" to "millimetres"?

--para. beginning "The supplied element":

change "The supplied element discussed in the previous section" to "The previously discussed supplied element" (it is mentioned but not really discussed in the previous section).

--para. beginning "The most common form":

change "The most common form of marking of text in manuscripts is" to "Manuscripts are often marked" (I do not agree that lines are the most common form of marking in MSS.)

--para. beginning "There will be instances, however":

"George Moulton-Barrett (Pierpont Morgan MA 310, Klinkenborg 23)": for consistency, place bibliographic details in the footnotes.

p. 434, para. beginning "As a rule, matter associated":

change "where these elements need tagging" to "where these features need tagging"?

p. 435, para. beginning "We repeat the advice":

change "the encoding of physical description of textual witnesses:" to "the encoding of physical descriptions of textual witnesses".

As a traveller longs for the sight of his country, so is the end of a work like this.