SOLVED!
documenting it here in case someone else has the issue.
if one can squeeze one's hand up in between the inner and outer fenders, there is just enough room to get the box end of a wrench on the fastener/nut. but, good luck trying to move the wrench more than an inch or less in such a tight spot.
so, I remembered that I had a many-toothed 'ratchet-end' box wrench. I was able to get it on the nut (after more attempts than I can even remember) and ratcheted it many many many times and off it came.
now, no way to get that nut back on (without taking the fenders apart), but I think the other two fasteners/nuts will do the job to hold it in place once I put it back after painting.