I'm fairly sure personality, memory, etc. stay with the mistwraith, not the spikes. Any hemalurgically appropriate set of spikes will do to restore a mistwraith to Kandrahood, and the resulting Kandra will be the same one that the mistwraith had been, not the one the spikes came from. That should answer your first four question, though I'll note for #2 that assimilating OreSeur would probably have made him actually less sympathetic to Vin - she'd been treating him rather badly.
For the fifth, we know dissolving a Kandra in acid will truly kill him. There may be other ways, and it's not clear how much acid is required. Hmm, now that I think about it, there may have been mention of a mistwraith inside the city shortly after the replacement, but my memory on that is rather unclear and I don't have WoA on hand to check. If my memory is correct on that, then OreSeur may actually still be alive somewhere out there, but by the time anyone other than TenSoon found out it was impossible to determine which mistwraith would be the right one to restore.