So since not all is written on this subject and we all have something to learn here, this online discussion helps us get our bearings straight on where we each need to improve. I can certainly take a few things from this exchange, myself. I've been inspired as of late to begin keeping stricter notes on the Coptic language in my personal offline database. It can't hurt, and it's already helped me gain a clearer picture.
One of the inconsistencies that popped up in the discussion was my understanding of the pattern of Egyptian noun-plus-possessive forms such as nb.f 'his lord' and rn.f 'his name'. I now perceive that Middle Egyptian nouns with long vowels must have drawn stress to themselves in these possessives, away from a penultimate default. So, while *rin 'name' with short *i leads to *ranífa 'his name' (nb. the preservation of Pre-Egyptian oblique case marker *-i correlating with that in Proto-Semitic), a word like *nība 'lord' must lead to *nībafa 'his lord' with accent on the long vowel of the first syllable (nb. the original vowel quality of the petrifact oblique marker is reduced in unstressed syllables). This seems to work well. For example, I account for the cuneiform-rendered personal name Bukurninip with *Boˀka-n-Ranífa [ˈbɔˀkn̩ɾəˌnɪfə] 'Servant-of-his-Name' (from an earlier *Bāˀka-na-Ranífa).
Another question I came face to face with as we were talking was: How precisely do vowels evolve from a Loprieno-derived model like mine to Coptic proper. After sitting on this for a few days while looking at a number of Coptic examples of my sound changes at work, I see that just a few minor modifications can fix things.
Let's say, as before, that by the 1st millennium BCE only Middle Egyptian long *ā leads to Proto-Coptic *o (nb. no phonemic length anymore) mirroring the contemporaneous Canaanite Shift in North-West Semitic languages. I maintain that short *a simply must have remained a in Sahidic in at least some cases, judging by how MEgy *sanáwi 'two' (rendered directly in cuneiform as ši-na-ah-wu in EA 368) became Sahidic snau. The AA cognates of this numeral only add to this likelihood. With *ā becoming a Late Egyptian *o, the selection of either omikron or omega in Sahidic (a matter of phonemic quality, not phonemic length) should depend only on whether the syllable was closed or open at the time, respectively. The presence of nasal stops just adds a slight twist by raising *o further to ou /u/, as in Sahidic noute 'god' < *note and moui 'cat' < *moya.
Likewise, as long *ī and *ū merge to *e (with loss of length contrast) this vowel must have similarly split into either eta (/e/) or iota (/i/) based on the openness of the syllable. However, in contradiction to my previous version of things in the commentbox, I now realize that I have it backwards. Sahidic i appears to correlate with *closed* syllables while ē (a front-high /e/ without phonemic length) matches best with open syllables in a later stage of the language. This then necessitates some interesting tweaks.
For Proto-Coptic *CéC (closed syllable), we then have:
- MEgy *pasīj 'nine' > PCop *pset > Sahidic psis
(Note EA 368 pi-ši-iṭ in cuneiform.)
- MEgy *maḥīt 'north' > PCop *mxet > Sahidic mxit
- MEgy *mūˁat 'truth' > PCop *méˀe > Sahidic mēē
- MEgy *rīˁa 'sun' > PCop *réˀa > Sahidic rē
- MEgy *mūḏa 'ten' > PCop *méta > Sahidic mēt
 I botched that up again! So sorry. I wrote "Sahidic i appears to correlate with *open* syllables". Please read the opposite. Sigh. Of all the typing mistakes, I make the most confusing one. LOL!