Hey Mike, I built two rudders. Looking back I probably shouldn't have, but that's another story
. The first rudder, I followed the plans and ended up right where you are. I think I used the blade of a wood chisel as a bucking bar. I may have even taped a tungsten bucking bar to the back of the chisel blade as close to the tip as I could to give a little more mass. This worked "ok".
I know we're geared to use the solid rivets at all cost, but if it will fit, I think I might try Turners idea of the blind rivet.
Not going to help you Mike, but for future rudder builders: Building my second rudder I waited rivet the lower rib together (using the 11 blind rivets) until I had those aft 9 rivets set (think I used a no-hole squeezer yoke). This way, I was able to separate the skins slightly and squeeze those tight rivets near the trailing edge. This worked much better.