Did do, yes. I believe that Oscar was also the runt of the litter, and wonder whether all his brain cells are intact! He falls over things, trips over things, instead of jumping he just kind of throws himself and hopes for the best. He's cross eyed, he has a soft palate AND a muffled heart beat. Does your kitty sound anything like this? If she's clumsy and not all there, for sure, get her checked out for other dysfunction. The vet reckons my little man is the result of inbreeding!! Anyway, I could be barking (or miaowing) up the wrong tree completely, but I just thought I'd give you a bit of background so you can see if there are any similarities
As for the bum issue, Oscar had a real issue. It wasn't 'wet' mucky, it was sort of dried on (
). The first stage is to get your moggy wormed if you haven't already. Worms do not help matters in this sort of circumstance. Anyway, I took to wiping Oscar's bum for him daily, he HATED it, he got very upset with me. So you've already tried that. I also keep a spray bottle of clean water, and if he comes in with a dirty tush I spray his bum a little and he tends to get the hint and clean it up. Some people don't believe in that kind of thing, but I don't see much of an issue.
Another thing to try (which is advice I got from this here forum!) is to change your cat's diet. I switched to only dry for a few weeks, at the vets recommendation. I tried the Hills diet (which folk on here don't like), he did well on it. But then it ran out and I was broke so I had to get a normal supermarket biscuit, but he did just fine! Now I've gradually added a wet meal as well as his biscuit daily and his bum is fine 80% of the time. I think he got the hint. I'm in the UK so if he comes in and his bum's dirty I leave him out for a few more hours and he normally sorts it out in that time himself. I haven't needed to wipe him for weeks.
Hope this gives you a few ideas!