[Corinthian]

1) No. Spells with the same effect never stack. Even if they are different spells. They do stack with items and innate abilities though. And items usually stack with other items not of the same kind.

1) Soulvortex does not differentiate between friends and enemies in any way. If you have two people with the same spell cast it next to each other then they would lose life over time but also lose fatigue. Might be useful in a communion if you have beefy slaves that also regenerates for some reason. Soul Vortex only heals half of what it drains (I think) but it also reduces the owners fatigue without reducing the fatigue of the one it stole from.

3) No idea. Probably the master. Not many self buffs scale with the casters power, so its not very important. Personal regeneration is the only one I can think of right now. And mirror images.

Edit: A quick test shows that the self buffs that scale, scale depending on the power of the slave, not the master. And no, theoretical extra slave levels due to numbers of slaves does not count.