I have a few questions....
1. How did an earring kill Vin's sister and make it through to Vin to give her the extra bronze powers?
2. What happens to the spikes in the kandra when they are being held prisoners, because i dont remember reading that they were in their pile of muscles but they also turn to mistwraiths if they dont have them?
1. The allomantic spike doesn't need to pierce directly into the person when that person is killed, it just loses some of it's power when outside of a body. Marsh's inner monologue tells all how it works when he's going to spike Lord Penrod. Also Vin's mother was under the control of Ruin, so the killing of her younger sister was probably pretty dang gruesome.
2. When they are being held prisoner they still have atleast the spikes that give them their mind, as when TenSoon is captive he mentions that he still has his. And yes if they don't have them they do indeed turn back into Mistwraiths, but would return to who they were when/if they were to get them back again, or at least that's my understanding of it.