I think the above is reasonable, though I could also see the Thonians as Oltec-descended and the High Thonians as .

Here's a more elaborate take on Wilderlands ethnicities: .

Possible origins of the Avalonians
While most of the Ordo Elementarum ended up hundreds of years in the future, some were flung thousands of years in the past, where they mingled their bloodline with water elementals and merfolk and founded the nation of Valon in the Wilderlands.

Alternate explanation: the founders of Valon were (very) early Alphatian planar explorers who had colonized part of the Elemental Plane of Water. When their settlement was conquered by (marids, later waves of Alphatians, the Ordo Elementarum, the hydrax, or whatever), they fled to the Wilderlands, where they intermingled with merfolk and founded the nation of Valon.

Doing some more research, it seems like the ancient Empire of Kelnore would be the most appropriate place for a Greco-Roman-style pantheon. They ruled just about everyone at one point (including the Altanians), but Tarantis is the location of what was once their capital. The Greco-Roman pantheon might mainly exist in Kelnorean ruins today, or the Tarantians might venerate it, or it might be more or less evenly spread.

I was trying to think of ways to link this to a Blackmoor/Wilderlands history. One thing that caught my eye was the Valoin, a prehistoric people who became the ancestors of both the Skandaharians and Valemen. While the authors' decision to make them Oltec rather than Neathar-descended was odd, I thought the Valoin might also be ancestors of the Avalonians and their savage Valonarian cousins.

Oh, I see. indicates that they assumed Oltecs were native to Skothar and Neathar were native to Brun. If anything, I'd assume the opposite.