Do these install under the singles or just sit on top of the gutters?

Bob, I believe you are over thinking how are attached, reason they go under shingle in the back is to hold them in place at rear as you install screw to keep it tied and in place on front. Now if you screw them in the rear you would be pulling shingle up and that wouldn't adhere to shingle above which is glued. That also gives support if you get a little weight on gutter guard with say moss esp if you are in NW when cleaning in spring. It;s a good standard design.

I have a 5" gutter but the guard is a little wider so it is virtually impossible to screw them in the back. It's to bad because I wanted them to work but they are going back. Not spending hours cutting these to fit. I could probably bend them but that defeats the purpose of the design.

These do not install under the shingles. They are installed on top of the gutter, fastened to the fascia and the gutter.