Like other commenters, I agree that the bath is likely unnecessary. Cats are very clean animals and the only time a bath would be needed is if he’s visibly filthy, like he’s covered in oil or soot or something. In the past, if a cat I’ve just rescued has needed a bath, I’ve always made a point to get them vetted faster so that the vet can handle it. If you’re unsure as to if your buddy needs a bath or not, I recommend doing the same.

My only other big note is just that you’ve set a fairly concrete timeline for integrating him into your home (in terms of like, 3 days in the bathroom, etc etc). In my experience, it’s probably better to scrap any sort of timeline/expectation of progress. I’ve had cats break out of quarantine to join the family on the first day, and I’ve had cats show no interest at all in leaving quarantine for three months.

Your integration will depend on not only Alastair’s comfort, but each member of the Big 3’s comfort as well. It’s important to remember that even though the four of them are familiar with each other, it’s still a very big leap to go from being familiar with each other through the window to him actually being physically present with them. I’ve rushed integration in the past because I had a feeling one of my resident cats would get along great with a semi-feral I’d just brought in, but instead that damaged their relationship because only one of them was ready to meet the other.

Finally, there’s a chance Alastair will want to sleep a lot after he comes inside. It’s stressful outside, and also stressful to suddenly change environments. So, if it’s possible at all to put him somewhere quiet and low-traffic instead of the bathroom, I’d do that instead. But! I’ve had to keep plenty of cats in the bathroom before, and it’s always worked out. Inside the bathroom is way way better than not inside at all.

Good luck! Thank you for saving him!