Yes. You can be the leader of the Catholic Church or Warrior's Hall at standing level 100+. You can be the leader of the Underworld too I think when you reach 100+ standing in Fenrik's Tavern but that reduces your influence per day(?). Other factions are capped for the moment.
I think you can but I do not know the exact requirements and it is not a guaranteed yes from them. Very low or 0 willpower will probably be needed.
To get the option, you talk to her, press Affect, and you can be presented with Enslave below the Convert option. On some NPCs, this option will not appear and you may need some required stat levels but I am not sure which.
I have not reached this point yet with my new accounts and in my old account I already completed Tales from Redhaven.
Apparently they can. Loren and Aiko at least gives me the option to enslave them. The roll will probably fail though and they will refuse at this willpower level. The main difference for Enya seems to be her willpower level but that's only at a glance and the difference is pretty small. Aiko has a lower corruption stat than Enya. They all have the same affection. Enya has 0 respect I guess but it is not like Aiko's is high either.