I want to talk about tempering

[Eisenhower]

Incarnate summonings pose another little piece of weirdness: not!Zenos implies that (an Echo-less, as far as we know) summoner that brings a primal literally into themselves will be 'tempered', and yet Yotsuyu acts enough like herself in her dying hours. Then again, the other cases we've seen of this, two of them died shortly after forcefully unsummoning the primal (Thordan, Yotsuyu) and Ysayle had an Echo/Blessing of Light bonus as well (so, by how -those- work, Ysayle wouldn't have been tempered anyways)

If you have a source for the "summoner that brings a primal literally into themselves will be 'tempered'" claim, I'd love to investigate for myself.

I'm ashamed to say that I didn't factor in Ysayle, Thordan, and Yotsuyu when considering the tempering process and effect. My instinctive reaction to it is that process of possession by a primal entity would change the target's aetheric signature in a temporary and non-destructive way. Or perhaps the changes to the host is different than those affected by a primal's tempering aura. Finally, it may be the case that the primal form is a sort of overlay on top of the host's form, like the shell of an egg and where the tempering effect radiates outwards only. There has to be some sort of control though. We know from geography (among other things) that oversaturation of aether can lead to permanent physical changes and it would be remiss to think that living beings intersected with primal aether would be exempt "in the wild", so to speak.

By whatever practical method, once the possession can no longer be maintained, the target's original signature could "bounce back".

I honestly don't recall the details surrounding Ysayle clearly enough, so I'm uncertain the nature of her Echo/blessing and whether or not she was in regular contact with crystal mom. Differences in tempering may very well stem from the summoning method used. To the best of my knowledge, all three possession-type summonings originated from Ascian plans - the summoners were either taught or provided with materials by Ascians. Thus the possession-type primals could be furnished with a different set of attributes and abilities based on the summoning method.