It turned out that our difficulties were caused by (again) sloppy previous maintenance. There were no less than six (!!) types of screws used haphazardly and seemingly willy-nilly on the starboard side, and some of them did not fit the lugs.
We finally got the tank off. Phew!
So one of our next projects is going to be the replacement of all screws and lugs to the two types that should have been used.
We're still having difficulties determining what screws and lugs we shold order. Everyone says something different. As the tip tanks were a post-factory addition, the Piper manual does not help. The screws we need are bigger and longer than the standard wing tip attachment screws. The jumble we found on our aircraft is of no help at all. We'd love to hear what the correct part numbers are.