If that were the case, surely they would've created mistwraiths instead of humans to begin with. Didn't Preservation specifically insist on creating humans with extra bit of Preservation in them? Surely, if Rashek intuitively understands mistwraiths in mere moments with the power, Preservation had aeons to think about them and decide against creating them. The "blockade between the Physical Realm and the Cognitive Realm" clearly was something Preservation didn't prefer, despite their peaceful nature.
I'd argue if agelessness comes from abilities that mistwraiths possess, then they should learn to use them intuitively over time, and that time wouldn't probably be that long even. There are real animals that are more or less biologically immortal and they have no mental capacity to speak of.
Well, he took away their humanity and their sentience, and reduced them to mere animals.
The first part makes least sense in all of this. How would a Blessing prevent normal reproduction, but potentially allow reproducing with another species? Surely, if they can transform their bodies to have functional human reproductive systems, then they also can transform their bodies to have functional mistwraith reproductive systems.
I'd argue if it's the same shapeshifting abilities, then mistwraiths should be able to use them as well, at least over time (like I replied to JustQuestin2004 earlier in this post). But your last quote kind of suggests it's the Blessings sustaining them magically. Blessings providing such magic I can get, but I didn't see anything about the spikes having such effect on anything else, which confuses me.