What would you recommend Macten? I had been starting to like the sound of the 'window trim' with tanking behind and siliconed top and bottom. Corner obviously a potential problem area, but I think if it's monitered and any failure rectified quickly then it could be the best bet. Can't raise shower tray as its a quadrant so impossible to fill the curved gap which would be left.
ps can't risk removing tiles as no spares.
Make sure you silicone behind the trim (If you end up going down this route), as well as top and bottom, if only to hold the trim in place.