For the merging of the Blades, I can see precedent for where that's possible. It's known that primals can effectively change the forms of their tempered minions (as seen by the pirate captain in Sastasha HM), so combining them into a new changed form is not out of the question.

Also, it was stated by Matsuno that had he known they planned on curing Tempering, he never would have killed off the Blades like he did. Yoshi-P and the other devs kept that fact close to their chest, and so Matsuno had us save the only Blade left unused, Lovro.