I was confused by that scene.Can Ranzkai transform Cadgan's physical body? Was it not the trick to show the reader the switch in personality?
If Ranzkai glamoured Cadgan's body everytime he needed to be physical with the badger, how did he teleport to his house at night?
If Cadgan saw badger being wrecked, why did he keep acting 'oh no woe is me' such star-crossed lover routine?
That's how I interpreted it. Cadgan's narration said they sort of have a brief moment of perfect synchronicity whenever they "switch control" is I believe how he put it, so ya it seems like one is always pilot, and one is always co-pilot, and the co-pilot takes that sort of ephemeral form while the other has the physical body. As far as the abilitiy to perform spells and teleport; I think you can glean something from the last chapter. Cadgan remarks that if Ranzkai can do it than somewhere inside him he should have that ability too. He mentions this when he's troubling over meeting with Silvareth, and how he'll have to hide the fact he has issues with his hands. Ranzkai's capacity for magic is immense; Cadgan's is so low that its even worth remarking by Hector, so it seems to be to quite an exception to even the average wolf. This is one of the many things that plays into their dynamic of Cadgan sort of lacking everything Ranzkai/Rian has in abundance, and vice versa. The shaman tree is a pretty clear visual metaphor for Ranzkai/Rian and Cadgan's dynamic. Why was Cadgan so woeful even though he experienced being with Hector vicariously through Ranzkai? I think theres quite a few reasons you can consider. You can make some inferences based on how Cadgan describes his perception of Ranzkai's memories. Where Ranzkai perceives them in vivid colour Cadgan experiences them distantly like a spectator. being with Hector through Ranzkai isn't the same as being with him as himself. For one they are keeping Hector in the dark so he doesn't even realize Cadgan is a part of these moments of intimacy. Cadgan is also more prescient of the rammifications, Ranzkai is able to lose himself in the moment Cadgan is plagued with the anxiety that this might damn them all. Cadgan realizes that he has lost a great deal of the qualities that Hector admired in him to Ranzkai, so he has this dilemma where he's only really half of the person Hector once loved. Again Ranzkai is not bothered by this in the slightest because he's more capable of just enjoying things in the moment.
Edit: oh one more thing Cadgan was still processing his hurt over Hector's lack of communication over the years. Ranzkai would experience these feelings, as Cadgan experiences his: sort of as a spectator. Ranzkai was able to pursue Hector without that emotional baggage hanging over him quite as heavily as it did Cadgan.