Clipping wings does not cause pain since all you are cutting is the flight feathers, which is about as painful as a haircut.
Just spread the wing feathers on one side only and cut them off close to where they are rooted. One side is better than both because it puts the bird off-balance and makes it harder for it to clear a fence.

I wouldn't worry too much about the muscovies getting tangled in avian netting... it is designed for birds, after all.

You MIGHT try, in the spring, gifting your SIL with two of the cutest little ducklings that your birds produce. Who knows, you might win her over. Be sure to tell her that if they don't work out for her that you will take them back.

Well, clipping wings didn't stop my drakes from flying, it barely slowed them down. Though I admit I clipped both sides, they were even enough to get over the fence.

When we moved, I used 3 stock panels, one on top, two sides to keep the flock inside.

We lost quite a few females to the road, they were flying low and got mushed by cars. So be careful, you may be liable for damages to the cars. I was always afraid that some person would get hurt. The wire topped pen stopped all that.

There is also Deer fencing which is enough to keep a flock in. I tied it with some string to make sure the flock saw it.

We are getting ready to make some simple pens with 1 inch plastic hardware cloth and 1 x 3 furring strips. Enough to keep everybody in place. The deer net will go over the top to keep the hawk out.

Last year she used our place as a smorgus board. I'm sure there are twice as many hawks in the neighborhood as last year.

Good luck
Kitty

Would making your run height higher help? Maybe add a couple feet to the top of it with chicken wire...this helped my problem..just make sure there's nothing for them to hop up onto and get over...mine found that jumping on the duck house roof would able them to get over,but when I moved it far enough away,they couldn't!!