The primal Shiva wasn't 'Ysayle combining with the soul of Shiva'. In fact it's an actual plot point that the soul of Shiva is directly accounted for and observed, and that she doesn't have it; Shiva's soul is still with Hraesvelgr. The primal Shiva is Ysayle summoning a primal just the same as any other, except in her case she believed herself to be the figure reincarnate instead of that figure being external.

Also, she transformed into Shiva that final time by using her Crystal of Light, which showed that she was in fact a 'chosen by Hydaelyn Warrior of Light' and not just a revolutionary cult leader. Again, this is an actual plot point. (She also didn't really destroy the airship, just damaged it enough to force a landing while failing to really harm anything or anyone inside, but that's a separate complaint.)


You are right, though, that the closest thing we've seen to the Arcadion's fighters before now is Reaper's Enshroud form, but the smaller details don't line up enough to use it as a comparison point. While we still don't know if the Reaper's Avatar is inside the soul crystal or if the soul crystal is a 'link' to the Avatar who's just off in the Thirteenth, we do know that Reaper is a stupid dangerous job and that the Enshroud form is the most dangerous part of it. It's largely for different reasons, though; the Arcadion's fighters can control those souls just fine and just happen to develop long-term health issues because of it, while Reapers constantly have to control the Avatar to stop it eating their soul, aether, and/or possibly body.