An 7.2-7.5 Speculation

[Vallavia]

I’m under the impression that on the surface Sharlayan and Amaurot using Greek is just because of the thousands of years of western culture associating the Greek language with knowledge. If you wanted to justify it in-universe, you could say the Sharlayans picked it up after studying the Ancients.

Not to veer entirely off topic, but I still believe that the original Archons who founded Sharlayan were either a) the actual Twelve in human form or b) acting on direct behalf of the Twelve, either of which put them on the trail of eventually making contact with Hydaelyn and thus solidifying the Ancient association (there's also the very conspicuous shots of the assembled Forum in their identical robes and obscured faces being direct parallels of assembled Ancients, but that could just as easily be entirely within the context of Endwalker's story).

This might get some more attention soon in the Occult Crescent with the accidentally leaked BGM making a tenuous link to the War of the Magi.

That said, though, I think I broadly agree with the 'on its face' interpretation you presented, the classical Greek association seems far more likely to be because it's suitably ancient and culturally ubiquitous. The Ancients/Ascians also fulfill a Titan-esque narrative role to the people of today; Hades even has a spell called 'Titanomachy'.

[MikkoAkure]

Not to veer entirely off topic, but I still believe that the original Archons who founded Sharlayan were either a) the actual Twelve in human form or b) acting on direct behalf of the Twelve, either of which put them on the trail of eventually making contact with Hydaelyn and thus solidifying the Ancient association (there's also the very conspicuous shots of the assembled Forum in their identical robes and obscured faces being direct parallels of assembled Ancients, but that could just as easily be entirely within the context of Endwalker's story).

One of the original twelve archons was indeed used as a basis of imagery for Rhalgr, but the same guy also defected to join the Ascians. Though that was 1.0 lore and I'm not sure how much carried over.

Relm was a member of the archons during this time period as well and she seemed to otherwise be a normal person so I don't think they're the actual Twelve. The Twelve also said that they don't interfere with the world.

[Vallavia]

One of the original twelve archons was indeed used as a basis of imagery for Rhalgr, but the same guy also defected to join the Ascians. Though that was 1.0 lore and I'm not sure how much carried over.

Relm was a member of the archons during this time period as well and she seemed to otherwise be a normal person so I don't think they're the actual Twelve. The Twelve also said that they don't interfere with the world.

I'm fairly certain that Ahldbhar has been mentioned again since then but not by name, though the imagery inspiration seems to have been retconned directly in favor of direct glimpses of the phantom realm (ostensibly the Ascian connection as well, but who knows?). I wish they hadn't, it was one of my favorite extracurricular lore tidbits (and could explain why Halone is consistently depicted as a Hyur and Thaliak as an Elezen).

I don't really think there's enough of Relm shown or described to make a determination either way, and I could conjure any number of characters saying something contrary to the things they do, most notably Hydaelyn herself, ie. their boss, who freely admits in her first direct conversation with you that she has repeatedly misled you or overly simplified her accounting of past events. The Twelve themselves also already lied straight to our faces to get us to do battle with them, which is admittedly a much different situation.

Granted, there's no reason for the Twelve to lie to us after Hydaelyn and Zodiark are gone and their missions fulfilled, but given their directive to guide and safeguard life on Etheirys I don't think it's out of the question that they might have given history a nudge when there's a particularly existential threat. The aforementioned direct Rhalgr sighting came after Rhalgr guided people to Gyr Abania with a comet. I'd call that an interference with the world, but Halone in her discussion of the matter with Menphina notes a delineation in their philosophy between 'aid' and 'intervening' in the affairs of mortals all while remaining unseen in their true forms. This suggests to me that certain measures of sending guidance or assistance are acceptable in certain circumstances.
I suppose there's a fair case against them being the literal Twelve, but certainly not against the Twelve's direct involvement or incitement of the post-Sixth Umbral Calamity recovery. Perhaps in the draft where one of their chosen Archons went off to become an Ascian they revised their policy on intervention in mortal affairs to be even more stringent, but who can say?