Speaking my thoughts about this.
If Amians can really transform into Larkins, perhaps they would need some sort of change in weather or enviroment, as the Listeners needed Highstorms to change form, though I'm not completely sure. Reshi was probably a huge change in enviroment from Aimia though, so maybe he was able to change on will?
Perhaps Vstim wasn't aware of Axies' presence in Reshi, and was simply coming for the Larkin corpse? As I think it is stated in the book, the Larkin was given to Rysn by the island itself, which if your statement is true, the Greatshell has some power in the Cognitive relm (or perhaphs it is actauly a spren, or being inhabited by a spren, this also leads to the question of if greatshell have at least some intelligence, or if thought is only given to these Huge greatshells. The Reshi believe these are their Gods, if that information is actually true, then I would assume that yes, they have thought.) If the Greatshell doesn't have power over the Cognitive realm though, this might suggest Axies' changed into a Larkin to escape and simply was hoping they would assume the excuse of the Greatshell giving a gift to Rysn.
Ah this is all so confusing.
