GURPS Powers, as usual, offers a few more options for Possession, including "Telecontrol 2" (+100%) which allows simultaneous but limited control over both the "home" body and the possessed one (basically, whichever one is given priority acts normally and the other gets a -4). The description mentions that Compartmentalized Mind makes that superfluous, though; wasn't Reia going to have that?
On a different note, I'm reading about Shapeshifting for Rose, and found something that may affect both her build and Howahkan's. GURPS Powers states quite clearly that "When designing Alternate Forms, apply enhancements and limitations to the basic 15-point cost per form but not to the form-specific cost (that is, 90 percent of the difference in template costs)." So a -50 percent limitation reduces the 15-point cost to 8, but doesn't do a thing for the 180 points that buys the 200-point template you shift into. Once I thought about it, that seems pretty fair, but it does make abilities for the major alternate form almost as expensive as just having them yourself (though, since you only pay the 90 percent for the most expensive template, it strongly encourages multiple forms!)
Powers also offers a number of very useful enhancements and examples for shapeshifting (as for many other abilities).
EDIT: EvilRoeSlade, sounds like Insubstantiality linked (+10% both ways) to Possession, probably with a mess of limitations since you won't be using it independently at all.