Agreed it is a massive loss of power for him. Cat's inferiority with her powers goes way back to at least the Camps, if it didn't leave her exposed it would make more sense to just permanently lend them out.
I would have agreed before the start of the recent storyarc. But she has just shown that while she lack advanced/technical skills then she is staggering proficient with the basic ones.
She did just kill a ranked Fae with his own sword.

I will concede that I am probably just straight wrong about things related to themes, since i seem to be dropping details.
The problem is its very hard to see what details are important, since there are so many

Nah, you can compensate for any of those. For example, the Legion mages lack skill but they compensate for it by just mass using a really basic spell. For that matter, Cat using Winter was very much someone using their Will and Power to compensate for their lack of skill.
For that matter, will and power seems pretty much the same when it comes to magic.