Hi Simon,
I have a Thomas Duplex safelight. I will run another safelight test tomorrow since I haven't done one in a couple of months. I'll also look for any darkroom leaks and reflections. I figured that if it were fogging of some kind, the exposure would be soft not hard edged like my second image. It also is not contained only on the edges. In the 4th image, it's in the middle and kind of big, like 1" x 2". I also made sure my tray was clean so there is no mucky residue. If you can give me any other tests to try, I will gladly do them because I would hate it if I returned a totally usable box of paper if it were preventable on my end. I really do appreciate your looking into it on your end. I think I'm the only one in the islands to use this paper so I have no one to ask about it. It seems to only happen to my 11x14s too, weird.
-lisa