I know we will find out what works best. But I think the wear on the fiber glass will be the problem if we have any later down the road. We will just have to be sure that bolts are good and tight on the fiber glass so that the bracket does not move at all. Now I know that you are glad that I started this thread. All the INFO in one place.
Having run fiberglass fenders on motorcycles eliminating movement at the mounting points is inherent to longevity of the fender. I used 3M 5952 VHB Tape between the mounting bracket and fiberglass and ran it the entire length of the bracket contact. It fills the uneven voids of the fiberglass and literally fastens the bracket to the fender when compress slightly.
From the pictures I have seen so far it looks like you only installing nuts and standard washers to sandwich the fender to the bracket. If this is the case in the long term the gel coat will crack at these points until the glass underneath deteriorates. Again my experience has been to dissipate the stress over a larger area. This could be done easily by providing a strip of metal the same width and length as the bracket underneath thus sandwiching a much larger area of fiberglass. Then add the same 3M 5952 VHB Tape to this metal strip to stop movement of the fenders. When you tighten the mounting bolts, do not over tighten or you will crush the fiberglass. Nylon locking nuts or Loctite should be used to assure the nut will not back off.
I have not purchased these fenders, but if I had the above suggestions is the method I would use based on my own failures I experience with fiberglass fenders. The sandwich method with 3M 5952 VHB Tape has been very successful. I however was not mounting from a single side as these fenders are mounted.