As far as I know, paper litters are safe. Some rabbits will eat it a bit, just need to watch if it’s excessive. Maybe put some fragrant hay in the litter box also to entice them to eat that instead. Some members have suggested buying the cat versions of these paper litters as they are cheaper than the rabbit ones!
You may want to consider covering the litter with a rack to discourage them eating it also.

As for your cat using the bunnies litter box, maybe put something in there that has a fragrance the cat doesn’t like but is bun safe? I’m thinking of a herb or something. Some cat owners here may be able to suggest something. Good luck!

If your cat is healthy and has had a negative fecal sample (should be checked yearly) and is strictly indoors so is not routinely exposed to intestinal parasites- there really is not health issue that I can think of.  If your cat goes outdoors- then you would want to keep your bunnies from being around the cats fecal matter as that is how most intestinal parasites are passed.

Behaviorally- I am not sure how most rabbits might react to another critter using “their” litterbox.  If they are used to the cat they might not think anything of it.  If my cats go into my rabbits cage or run- they follow the cat closely with an “excuse me!” expression on their face.  They are not afraid of the cats, but they are not super impressed by them either- and one of my buns even will bully the cats.