This evening, after working on a few other alterations, I sliced through the stitches on the third, lower strap on my Stillwater wool kilt. It was held on by a single row of stitching and between the double row of fringe. BTW, I did have to sew that back closed.

I put on the kilt; put the strap through the buckle on the first hole; then taped it in place on the underapron.
After taking the kilt off, I sewed the strap in place using the original holes, then I put the kilt back on.

It worked! The third strap is no longer attached to the top apron and pulling on it. It is now harmlessly attached to the underapron and pulling on nothing.

When I first thought of doing this, I was worried that the top apron would move away from the strap when I moved or sat down, but it pretty much stays in the same place. It seems like that is because the strap is a little above the bend of the hip, and the top apron folds right along with the underapron.

This is temporary until I work out a way that I can make the strap removable, so that I can switch between brown and black. It would be better if the strap were a couple of inches longer, but the top apron does cover the end of the strap. I also need to tack down the end of the strap that extends about an half inch passed the stitching.