Panacur (Fenbendazole)is very commonly used and seems to be veyr safe (I've used it on all mine at least 5 times, I do it regularly) and have never had a problem.
The Droncit is not commonly used on rabbits. Tapeworm is not known to be common (although I have wondered if it was more common than previously believed because I suspect people maybe miss it, I know I would have had it not been down to Angieluv) and so the treatment is also not common. My vet had never had it happen in a bun before so had to talk to the drug company as to what was best to do (We went for Droncit injections, and each of my 17 had two injections each, with no side effects other than a thirst immediately after). So this is not done at a specific time, and most vets would not even consider it, but that does not mean that it should be ruled out, and it may be the cause of gut problems (it did in Summer, my foster, although not anything like you have described, she just got excess cecals VERY easily). Might be worth a think about. I believe Angieluv used Droncit tablets with her buns. We steered away from those because my vet felt he would prefer to be more precise with injections (but that was personal choice, so knowing about tablets does give you another option).