It's wonderful that you rescued your second rabbit and are planning to bond her to your other rabbit!
A few things stuck out to me from your post, and I'll explain why:
1) Your newly rescued female isn't fixed yet, and you're doing slow introductions.
- It may be more beneficial to keep her isolated and let her become comfortable in your home at this point, rather than slowly introducing them. It's not as productive as you may think to begin introductions before she's spayed. After she's spayed, you will need to still wait about a month to allow her hormones to regulate and for her to fully recover without any interaction with him anyways. I recommend stopping the slow introductions and focus more on letting your new female be comfortable with you and her new environment.
2) The rescued female will be spayed next week.
- I think it's great that you booked a spay so soon after getting her! It's really important health-wise for a female, and also for the bonding process. To reiterate, once she's spayed, it doesn't mean she's "good to go" with being bonded with your male. You will need to wait for her to recover.
3) The male does not use his litter box.
- This can actually be continually happening because of the introductions you're making. Although your male is neutered, the presence of an unspayed rabbit can actually make your male act hormonal again. He could be marking his territory in response to a foreign rabbit. Outside this, I'm curious about what techniques you've tried and the set-up of his litterbox(es).
4) You'll begin water bottle training with the male and the cat.
- I'm unfamiliar with this phrasing, but I assume you mean using a water spray bottle to stop the bunny and cat from being aggressive with each other? Please let me know if this is wrong. Based on that interpretation though, I would strongly advise against it. It's very unlikely that your rabbit will understand why he's getting sprayed, and will most likely just associate it with the rabbit, fueling his aggression. Cats are natural predators to rabbits, so if your male is one who is aggressive towards your cat, that is unlikely to change through training because it's instinctual for your rabbit. It may be something where, over time, your rabbit learns your cat isn't stalking him (if your cat is, you definitely need to separate them). It would be best to minimize their interaction time for safety's sake.
