The fact that Khriss mentions an ocean in which Foil is studying how to control the Aethers and later in the chronology we see Xixis living in an ocean where he studies the Aethers and shows he can control them seems to me to be a clear allusion to Lumar being the place where Foil/Xixis is at the moment that ars arcanum is written.
Secondly, while it is true that Xixis without control of the Aethers SHOULD not be able to make his palace Khriss specifically says "Foil, deep within his ocean" that to me sounds like he is already installed in Lumar probably not in the same way that we see in Tress but it is on the planet.
But what I wanted to discuss is not how or when Xixis learned to master the Aethers but rather if the Iriali leave Lumar not long after Xixis's arrival and he was already there by the time of Rhythm of war then those iriali cannot be the same as those in Roshar.
I'm also not very sure about the idea that there are several Iriali groups but it is the only conclusion I could come to, although your idea that Xixis could be studying the Aethers on another planet before Lumar seems to me to be able to answer my doubts. It doesn't quite add up to me because of the issue I mentioned before, which sounds too similar to Lumar in the ars arcanum, but it does seem like a good answer to me.
Regarding the supposed Iriali of Scadrial, if they really are, it doesn't seem to me that they are a third group because those do fit with the chronology if this is their next land after Roshar.
Yes, you're right about that, I just wanted to talk about this and see what conclusions other people came to.