My comments, all in theory:

Generally, I would say engine's torque and power curves are fixed. The details of the prop are irrelevant as far as torque is concerned. The rpm is all that matters. If you put on a bigger prop load (pitch or diameter), and retune the engine for max performance (i.e. the needle)... the max rpm will be lower, so will the power, and so will the torque.

That assumes that you are operating beneath the engine's peak torque.... which is probably the case. If not... well then you may see a small increase in torque before you see the decrease.

Bounded by those constants... you pick the right prop. Since you're hovering... you do what people have already said. Larger diameter and lower pitch.... if the engine spins slower than you're torque is lower.... question is: Do you still have enough thrust... have to experiment.

If you want to try something a little wacky, you could try slowing down the roll rate of the plane by putting some weight out at the wing tips. At least, in that case, it will take longer to "come up to speed" when rolling. It would degrade performance in other areas I suppose.

How retro fitting about a tail rotor?

Split the elevator and run then reversed (a bit) to control roll?

Tiny retractable parachutes (or something) from the wing tips to add drag?

Dip the tail fin in water while hovering. That'll do it!