I have no problem with PvP in the jumping puzzle, however I think there should be a way to complete the puzzle when out numbered.
I had this issue the other day. There was a constant fight in the area just before the dark room, 20+ players from opposing teams fighting each other. I was alone and got slaughtered the moment I entered. I returned but used the invisibility fountain before this area. It took me a number of tries to get through due to me getting caught in cross fire and loosing my invisibility. I eventually made it into the dark room and after getting through there I got to the circular room only to find it camped! At this point I gave up.
I actually found trying to negotiate my way in invisibility through a room full of cross fire quite enjoyable and very challenging. I think a fairly simple solution to jumping puzzle griefing would be to allow for invisibility to be available in the circular room. This could be done by extending the duration of the previous fountain or by adding a new one.