If he was born this way, he has genetic splay leg. If he wasn't born this way but has been this way since you got him, then it's possible he incurred an injury from the cat attack. Either way, slippery floors could be difficult for him to manage on. If he is able to hop or walk pretty well on non slippery surfaces, then it will probably help him to not be on slippery floors. But if he mostly scoots even on the carpet, and he can scoot easily on the slippery floors, then smooth flooring may actually be helpful to him. I guess see which flooring surface he does best on.
Here's some info on splay leg and some ideas that might prove helpful. If your bun has perfect function of his front legs, some disabled rabbits do well with a cart that can support their body and hind legs. Might be something to consider for your bun.
http://www.disabledrabbits.com/splay-leg.html#:~:text=What is Splay Leg?,it away from their body.
http://www.disabledrabbits.com/wheelchairs.html
The splay leg is not likely correctable at all at this stage, though a good rabbit vet can help show you how to manage issues that could arise from the scooting, such as leg sores. If a messy bum from urine or poop, continues to be an issue, having the vet clip the fur around the butt would probably be a good idea. It will help in keeping the bum cleaner, which is essential with the coming warmer months. You will need to regularly check his hind end, as a wet butt from urine or mushy poop, will attract flies and put him at risk of developing flystrike, which can prove fatal to rabbits in less than 24 hours.
https://rabbitwelfare.co.uk/rabbit-health/illness/flystrike/
https://wabbitwiki.com/wiki/Flystrike
https://rabbit.org/vet-listings/
For the poopy butt, I would start with removing the apple and any other high carb/sugary treats. If that doesn't resolve it after a couple weeks, next I would remove the alfalfa hay, which would need to be removed anyways if he's getting close to or over 6 months old. Though in removing the alfalfa hay, you'll need to make sure he's eating the timothy hay really well from the start. You just don't want to pull the alfalfa, then have him not eating hardly any hay at all. If he's not eating the timothy well, then you may need to do a gradual reduction of the alflalfa over time, so he gets used to eating more of the timothy. Then monitor body weight and body condition to make sure he doesn't experience any unhealthy weight loss from the diet changes.
Then see if the poopy butt improves after that. If not, it could be a sensitivity to the veggies, too many of the greens, or even a sensitivity to the pellets. I've had rabbits in the past that couldn't have any fruit or very many pellets, or only certain types of pellets. Even some that couldn't have any pellets in their diet at all. So it's just a matter of seeing what your rabbit is sensitive to.
