Well, jumped in there with both feet AGAIN! After very careful inspection with a telescoping mirror and all, I think I found how it's held on -- a small U-shaped wire clip (less than 1/8" across). With a miniature screwdriver I was able to get under that clip (with the mirror in the other hand and head upside down) then drop the mirror and use a large, square-shafted screwdriver to try and pry the top and bottom edges. After an hour and lots of WD-40, still not budging, but I do see if it wasn't frozen by evidently salt corrosion, how it can come off.
New tactic, undo the nut holding the shaft just enough to cut the old o-ring out, buy an o-ring at Ace Hardware and slip it over the whole arm and shaft (after removing the wiper blade) and let it shrink back down around the shaft. If I ever have to remove that arm because something broke, I'll get a bit more violent with it, can't lose at that point...!
Thanks all for your help, great to have a large team working with you.
Ron, yes the center wiper works great, it rotates (lifts) up when the center window is opened, no problems. Although, there are about a million different p/n's that AFI sells, I ended up buying their new larger plastic arm assy which blocks the view slightly more than the metal -- they evidently discontinued the metal ones(??). Only tricky part was that the shaft was like 2 1/2" long (originals were much shorter, but again, couldn't seem to order them). So I had to make a spacer block on the inside to position the outer end of the shaft closer to the boat's exterior surface to avoid interference with the overhanging brow with the window in the up position. If you like, I'll take some closeups of the block installed. It really makes a large difference to have that viewable area while underway in big water/waves. Very well worth it.
Steve