They are all splinters. All the evidence needed is on the coppermind here. I can find wobs too if you'd like.
That is completely fair, but kinda irrelevant. At the time of the machine, virtuosity had splintered there. Otherwise, the huge amount of spirits there would have either never been captured, as virtuosity would probably never have allowed such a violation, or they wouldn't have been there to begin with.
I'm not sure what you mean here, but technically it didn't kill every human; otherwise, where did Painter come from? It drains easily accessible investiture, like people's souls. I don't think Hoid counted as a human for the machine; more like a spirit. (Hoid Hion!?)
Do you mean the Heralds or Yumi? Painter shapes Yumi's investiture into Yumi again, and the Oathpact shapes the Heralds back into Herald shape. They aren't dead, just gone from the physical realm.
It's a normal thing, then extended by more investiture.
It used the investiture of souls to kick start it's process. Once it absorbed the souls, they were kinda part of the machine and would stay around as long as it did. The Machine keeps them in the physical realm, and the investiture to do so comes from the spirits.