The schools come later I guess I failed to specify the timeframe for that.
It's more or less a given that it would take centuries to have something similar to a complete education system ITTL. Meanwhile, I think we can agree the linguistical development would continue.
And when the situation would allow this (which is not the same to say it would happen then) the devellopment of Vulgar Latin and the need of basic efficiency in at the very least day-to-day administration wouldn't make Classical Latin the new standard.
What I was referencing with a unified state is that its presence could be enough to keep regional speaking changes 'dialectal' instead of them developing into their own separate languages.
Thing is, these supra-dialectal groups were issued from Vulgar Latin rather than Classical Latin. While I agree the linguistical differenciation would be more limited ITTL, regionalisation associated with a diglossic relationship with Vulgar Latin
s and Classic Latin would still be a thing, mostly because it already was the case IOTL since the IIIrd century.
I was thinking the present of the state would do a lot to pull people together and around the more so than in the fractured kingdoms of the 'Dark Ages' allowing the mixture of the mixture and transfer of larger dialectal changes across superregional lines i.e. there being a type of Western Latin spoken in Gaul and Hispania, Insular Latin spoken in Britannia etc instead of the even greater regional breakdown into languages and dialects like Occitan, French, Arpitan, Catalan etc.
Regionalisation was more or less a feature of Late Roman societies, arguably possibly more in some as Late Roman Britain. I think we're still bound to see a differenciation happening very roughly along the lines of supra-regions such as Gaul, Spain, Britain, etc. And I agree that the linguistical differenciation in modern Romance language would be considerebly butterflied away (see my earlier post about Germanic influence o French, or Vasconic influence on Castillan and Gascon).
This being said, you're bound to have some Germanic influence there and there in Western Romania, not only due to laeti presence, but the foedi that are bound to be preserved there and there (the idea of a ethnic cleasing with Romans pushing Germans back to Rhine is, frankly, ludicrous : it makes little political sense for the Vth century). Let's take a PoD where Constans III becomes emperor, so it means Frankish influence in Northern Gaul, Alamans in Middle Rhine basin, possibly Gothic in Aquitaine, etc. Giving that reducing these entities into semi-autonomous ones would already be a challenge, we can consider that they would form possible linguistical particularities : maybe some *French in Netherlands, *Alemanic/Burgundian in Upper Rhineland, etc. I'd expect as well some influence on super-dialects (such as these two or three on Gallo-Romance), to say nothing of a likely Gaelic superstrate in Britain, assuming a Romance language really manages to blossom there and not just an extension of Gallo-Romance.
Basically fewer distinctive dialects than OTL
I agree, up to a point : the linguistical distinction between, say, Old French and Old Occitan is a relatively late happenance from the Xth century, but it's due to the fragmentation of political, administrative and ultimately cultural centers. ITTL, you'd probably have a relatively less complex distinction within these super-dialects without real way for these to become more. In short having a southern Gallo-Romance speaker speaking something essentially similar to a North-Western one (with the caveat of aformentioned "foedi dialects")
Scholars, essentially, on the lines of "muh Latin".
If it was wished that the writing style of officials ecclesiastical and secular was more brought in line with on ground realities (with the support of the church of course) could we see written Latin be brought more in line with that of speech?
If the problem was essentially phonologic, I would agree : but the evolution of Vulgar Latin was (as we saw) a matter of grammar and vocabulary as well as phonology. It doesn't help you already had regional variations already in phonology at this point (such as Vulgar Latin in Gaul loosing open vowels, and many others such as loss/gain of diphtongs). At the very best it would be a cosmetic adaptation of Classical Latin on Italian speeches (which were the closest, arguably, to Classical Latin).
Except as a very first step into "vulgarisation" of administrative/institutional languages (which would be, I agree, an interesting development), I'm not sure it would do much.
Of course, there are those writers outside the state and church apparatus but with full support from both institutions, could you?
You mean an actively supported script change? It seems a bit...convenient, giving that you didn't have such concept in Romania. The inter-regional exchanges were enough (and would be enough ITTL) to switch to new practices IMO.
Also while mandated schooling for the masses is practically impossible for a pre-modern state
It's arguably hard enough for a pre-modern state to go for a middle-class schooling : medieval universities really beneficied from the rise of medieval bureaucracies both clerical and royal. There is a reason if the intellectual is a characterization which is medieval and not ancient.
mandated schooling, in general, is possible for a pre-modern state to force upon the higher echelons of society as seen in the Education Act 1496 of Scotland.
To get a bit technical, by the XVIth century, we can talk of early modern states in Western Europe.
Now as for the causes and consequences of the act, it was a consequence of the humanist thought in Scotland (arguably an early one) and the necessity for high classes to have a formal teaching on humanities.
Giving the decentralized, and relatively privatized, school network in the Empire I'm not sure such act would be needed (more or less as you pointed yourself) : the imperial and provincial courts recruited among the same elites which generally took on themselves this education.
Now, in case of an important crisis (let's say a TL where WRE survives in the mid-Vth but is forced to abandon most of direct rule of provincial areas to foedi, which it slowly re-integrates), you could see the introduction of a standardized cursus for what matter administrative tasks, but it would be a continuation of the late classical paieda rather than the less elitist early medieval crusus which tolerated "Rustic language".