So, it seems likely that the oldest amphidynamic pattern was é/ó-ø-s ~ ø-é-m̥ ~ ø-ø-és (nom ~ acc ~ gen (all amphidynamic nouns are in fact masculine). The newer hysterokinetic and amphikinetic patterns are offshots from this, with various further developments. Therefore, I'm going to take Methinat from a region that never underwent those developments, to make things easier (although, in the long run, I don't think it much matters). Now, this pattern does itself have an interesting two-fold development in Methinat. The three major developments affecting this are: appearance of ə from syllabic consonants; generalisation of the nominative stem to the accusative; generalisation of ə to apophonic zero-grade syllables. These three developments, ordered chronologically, yield the following two patterns: é/ó-ø-s ~ é/ó-ø-əm ~ ə-ø-és (in stems whose suffix does not contain a syllabic consonant in the nominative); and é-ə-s ~ é-ə-əm ~ ə-ə-és (in stems whose suffix does contain a syllabic consonant in the nominative). Thus, we find *déh₃-tr̥-s ~ *dh₃-tér-m̥ ~ *dh₃-tr-és "giver" > dóʔtərs ~ dóʔtərəm ~ dəʔtərés on the one hand, and *déyus ~ *dyéwm̥ ~ *diwés "sky, sky god" > déyus ~ déywəm ~ dəywés.
The proterodynamic pattern was clearly é/ó-ø-Ø ~ ø-é-s (nom-acc ~ gen (all proterodynamics are in fact neuter)). The unique s-stem ablaut is a product of later developments. Unlike the amphidynamic ablaut, proterodynamics all develop the same way, to é/ó-ə-Ø ~ ə-é-es, as in *dór-u ~ *dr-éu-s "tree" > dórəw ~ dəréwes. s-stems, however, all gain a stem alternation between -s- in the nom-acc and -h- in the oblique. Thus: *nébʰ-s-Ø ~ nbʰ-és-s "cloud" > *néβs ~ *nəβéses > néfəs ~ nəféhes. This adds them to the ranks of inherited heteroclites (the numerous r/n-stems, and the occasional l/n-stem). An interesting case is *gʷenh₂- "woman". Szemerényi's Law gives *gʷḗn ~ *gʷnéh₂s, which could easily survive as gḗn ~ gənáʔas, with a synchronically irregular nom-acc sg.
The static nouns are awful to try to reconstruct, but I think it likely that there was a class with ó/é ablaut, though not ḗ/é ablaut. That "Narten" inflection never worked for me. There may have been a class with no ablaut whatsoever, but that doesn't sit right with me either (though, admittedly, it is more likely than "Narten" inflection, given that the two mobile paradigms could have either o- or e-grade roots in the nominative, so it's not out of the question that the static paradigm could too). Anyway, based on what we would expect based on the mobile paradigms, they would probably have this pattern: ó/é-ø-(s) ~ é-ø-(m̥) ~ é-ø-s > ó/é-(ə)-(s) ~ ó/é-(ə)-(əm) ~ é-(ə)-es. A good candidate for a static paradigm is *nógʷʰ-t-s ~ *négʷʰ-t-m̥ ~ *négʷʰ-t-s > nówts ~ nówtəm ~ néwtes.
Next up: Syntax!
I actually got a lot of my ideas from reading about Tocharian. The role of ə specifically, as will be expanded upon soon, was shamelessly stolen straight from there.
Yes, that's the outcome of the palatalisation, with ś coming via ź, with the same development of the aspiratae to fricatives as Italic.
In regards to accent, I'm a little torn as to exactly what to do. Having read many of Kloekhorst's articles on the accent-ablaut system, I've been forced to abandon some notions, while others are essentially confirmed. The only differences I have in opinion with Kloekhorst are (as always) in the specifics. The options are mostly where down his proposed line of development in the accentual system to break off. Methinat has always been roughly contemporary to Anatolian (in terms of detaching from the family) so it's reasonable that the inherited systems will be similar. Or, I could make Methinat even earlier than Anatolian, in which case I could just straight-up start from the primordial system with no additional changes. But that then adds the problem of assigning lexemes to this system.
That sounds wonderful! I always like to cross-reference the dictionaries I have with one-another, to see where the authors agree and disagree with each other. It's far more enlightening than just being exposed to one viewpoint. I also plan on using said dictionaries to make my own PIE lexicon, mostly for conlanging purposes.
Ok, now for the nominal morphology. I'll leave accent-ablaut for later, since it's such a difficult question (and there are lots of difference answers to consider). For this post, we'll do endings.
The surviving cases at this point are nominative, accusative, genitive, dative, locative, and instrumental. I don't believe that PIE had an ablative case, and the allative was apparently already on its way out. The PIE terminations were as follows:
The full-grade alternants of the ablauting endings are generalised through athematics, and with regular sound changes this yields:
And here's what the system looks like by the merger of o into a:
I'll finish this post with a discussion of the neuter endings. It's fairly uncontroversial that the neuter didn't originally have a plural, and that one was created partly from a collective (providing the nom-acc plural), and partly by importing the masculine endings (providing the remaining cases). In Methinat, the nom-acc was instead created by an analogy with the vocative. In the athematic singular, both were endingless. On the basis of that identity, the vocative plural -es (which happens to be identical to the nom.pl.) was imported to provide a neuter nom-acc plural ending. For the thematics, the same case ending (in this case, -oes > -ōs) was imported. The other cases were simply imported from the masculine. At some point after this, the vocative then merged with the nominative.