I don't buy the mistwraith theory at all. It is wishfull thinking.
I doubt there are any mistwraiths post-catacendre. Harmony presumably restored them to people.
Even if you posit some occult hemalurgic bindpoint on an essentially formless creature, why should this be any more effective than say, spiking a cow? Per MeLaan, biologically speaking humans are much closer to cows than mistwraiths/kandra.
Which gives me this funny image now of Spook stalking the plains of the Elendel basin hemalurgic spike in hand, trying to stab a wild cow in the eye, getting chased off by bulls, Beldre increasingly questioning her life choices...