Invocation

I'd keep it as Kel-see-er, since if I'm remembering the WOB about the pronunciation right the Kell-see-ay pronunciation is the noble way to pronounce it and since the majority of the cast is skaa and/or actively embracing their skaa halves over their noble halves, it makes more sense in my mind if they used the skaa pronunciation.

The Tranquiline Halls is Ashyn, another planet in the Rosharan system. Vax is something different.
Plus, Ati is not from Roshar, as there were no humans on Roshar until after the decimation of Ashyn (which gave rise to the myth of the Halls), by which time he was probably already trapped by Leras on Scadrial.