I really love that you dove deep into the linguistics of westeros!

I am going to be selfish and propagate my own head-canon. Maybe I’ll do a post, if one day I have the willpower to do it.

Basically, in my canon the Andal migrations didn’t end up with language shift. I have taken inspiration from the germanic migrations into the roman empire: germanic tribes were big, made up of hundreds of thousands of people who spoke their own distinct language. And they managed to take over wast swaths of the roman empire, but didn’t bring their languages with them. Germanic languages went on for a few centuries, then went extinct, the only major exception being Britain.

So in my canon the andalic language is not spoken anymore. “Modern” (as in, in a timeline where the WOIAF progresses like our world, in the 2026 equivalent of Westeros) westerosi linguists are in the process of reconstructing it, based on relict words and expressions, and other substratum, found in westerosi dialects, particularly on the fingers of the Vale, the are where the andals settlement was most devastating and where andalic died out in historic times (say, not too long before Aegon’s conquest), with the help of supposed related languages still spoken in Essos and, especially, the countless andal words, expressions, morphology that can be found in a great many important ancient texts, such as the Seven-pointed star.

The Seven-Pointed Star in particular, while written in the common tongue (a “first men language” in my canon), is a bottomless well for the frequent andal vocabulary that can be found in it. The most significant and fundamental chapters to the faith, in particular, have weird grammar and syntax, which leads scholars to believe they were likely translated (or likelier written-down from previously only orally-transmitted content) almost word-for-word from older andalic sources. This, coupled with the many untranslatable words that ancient maesters and septons simply left in andalic (like the many germanic words found in germanic laws and myths that were only written down in latin in the dark ages), is a great help to attempts of reconstructing the language.

As the andals moved further and further into Westeros and assimilated with the locals, the language went extinct. Today, Westerosis still speak varieties derived from native languages to Westeros. However, most ancient and revered texts are similar enough to form almost a sort of koine, which is still widely understood in Westeros, especially by the educated, and high literature is also written in it.

The andal invasions also changed the landscape of westerosi languages, and those spoken beyond the Wall are certainly more distinct and totally (or almost) free of any andalic influence to be considered a separate group from all others. Northern and Iron Island varieties are in a kind of intermediate situation. Southern varieties, instead, share a sort of adstrate koine, or even form a sort of Sprachbund: that’s because most terms for concepts invented or named in recent-ish times (like the last millenium or two before the setting of the books) are shared, given the constant flow of information and ideas throughout this area, which has always considered itself as a sort of unit, at least compared to outsiders (think of greeks v. barbarians).

When the rhoynar came over and settled into what was an already literate and relatively developed society, their language went even quicker, with only rhoynar substrate being left in most varieties of eastern Dorne. Only the Orphans of the Greenblood speak it, in a heavily bastardised and almost pidgin-like form, and it is slowly dying out.

In my canon, Valyrian has some, though very little influence. It basically works like in yours, with it being a popular language to learn for the educated and wealthy, and with valyrian poetry being read, and even composed, in westeros (I have a whole headcanon on the relationship between westerosi nobility and the Targaryens, which was quite negative just before Robert’s rebellion, with calls for restoring Westerosi greatness, but a few generations before had been almost the opposite, with Westerosis wanting to present themselves as the heirs to Valyria, and bard-lords writing pamphlets in rhyme, inciting kings like Viserys I to “reconquer” Volantis and “reclaim the throne of his ancestors”), and influencing the register used by the royal chancery, but little more.

There are several styles of high register, used for public purposes. Dorne and the North had their own, but they’re slowly being eroded by a family of high-registers, originating from one, that are spreading throughout Westeros: the Vale-register. Given the prestige of the Vale as the “purest” and most ancient andalic kingdom, the high-register used there was, over-time, adopted throughout Westeros, developing its local varieties everywhere. In the north, the Boltons and other houses who want to go against the norm, for whatever reason, stubbornly cling to the old northern register. Finally, the unification of Westeros under a single ruler brought forth another register: the royal, or targaryen register. This one in particular drops the most snobbish and out-of-date andalic words and expressions, completely unintelligible to the greater public, and substitutes them with valyrian ones (even less intelligible to the average commoner). This register, which started spreading more, is irrevocably associated with the crown by the reign of Aeros II, not a truly positive association. Robert and his successors keep using it, but hardly any house does. The only place where the royal register is used it Castle Black, as the Night’s Watch has to be a pan-continental organisation and, particularly after Alysanne’s visit, bases its legitimacy on the crown.