and also with the appreciation for Captain Ferrers, in whom I wonder if we have a more positive example of a slightly effeminate male character than Greening?

Oh, that's true! He's not explicitly described as effeminate, but "elegant" and "exquisite" do at least make him sound like a dandy. Also he says "la". : )

Their shared Highland identity, opposed sides in the war, historic clan enmity and the present imbalance of power from Ewen being Loudoun's prisoner all combine to create a complicated dynamic.

Yes, it does make for a complicated situation--they have a shared cultural understanding, but also a lot of things dividing them.