Character build help: David Haller (Legion from the X-Men)

[Culture20]

David Haller is the son of Charles Xavier, and himself an omega-level psychic mutant. He possesses the ability to create within himself numerous other-selves each with their own psychic power. Unfortunately, his other minds are full blown personalities with memories, goals, and emotional states of their own. Some are created as copies of people who have died in David's presence, and thus are "real" people with newfound psychic power "trapped" is his body.

At first glance, Legion is a simple build: cosmic style modular abilities advantages only, with mutant and psionic power limitations, maybe some comparmentalized mind requirement, toss with a light vinaigrette and liberal sprinkling of mental disadvantages, and you're done!

Except, David has many ways to activate his powers. At the height of his power,

Spoiler:  

he had merged all of his minds together. This would work as above with far fewer mental disads.

But usually, he has to either dominate a personality and assume its power through psychic combat, or convincing it rationally. The danger is that the other personalities always want control of his body, and if he fails to assert control, the personality in question can gain control. Sometimes, personalities will fight each other for control too. He can, in a pinch, allow a personality to control his body if he knows the power is needed but the personality is too strong, but then the personality gets to decide everything from that point, and they rarely care to help.
A third way, is that he has shown tehe ability to corporealy manifest a personality outside of his body, giving it an independent existence. Sometimes he has been able to strike deals with the personality to make it do what he wants while it is free (the deals usually involve further freedom or bodily control, but can include anything the personality desires).

The psychic combat or social engineering could easily be a mere skill roll, but handling the failure (or surrender) is where I fall short. Rule 0 makes it easy: "the GM says you temporarily have the following disads and personality: role play them" but what sort of limitation discount would that be for the base advantages?
The Separate manifestations? Summoning a person who might be an ally, enemy, or escapee, depending on how the rolls go. And once that personality is gone, he loses that power until reclaimed. Again, I'm at a loss as to how exactly to model this.