3. It's just Sponsored Magic, but the compels come from an internal force instead of an external one. I figure that you could use standard Sponsored Magic mechanics without any trouble.
4. Any time you could spend a FP, you can take a point of debt instead for the same effect. Would probably carry thematic limitations; maybe it'd only work on High Concept invocations. Not sure how much it should cost.
3: agreed. Got a PC using this in my game, and it works just fine. (Edit: though I'll admit that part of the reason it works is that the player has discovered that I have no qualms about doing horrible things with compels, and is thus suitably wary of racking up too much sponsor debt...)
4: Hm. No more than -1 cost, I'd think; it's really just an alternative version of demonic co-pilot. Possibly -0; I mean, it's not like you're getting a free lunch anywhere - and there are a number of other examples where a PC is allowed to "borrow" fate points that they don't have. (The two examples that spring to mind are: temporary powers when you're out of fate points, and boosting the power of a potion.)
I could easily see this as being rolled in as part of a Sponsored Magic, actually; a number of the more narrow ones seem to offer extra bonuses of some kind, and this sort of thing could fit right in.
