Need advice re: four cats

I’ve always had cats but I’m in over my head.

We have had two cats, littermate sisters, for a couple of years. These are cats A and B. A is the dominant cat. No issues.

About two months ago we found a kitten outside. This is cat C. He was healthy, no microchip, 10-12 months old. We got him neutered. A and B aren’t big fans of his, they fight almost every evening, but everyone was eating and sleeping well, pooping and peeing where they were supposed to.

In the last three days cat C has been peeing on beds. I took him to the vet and they found that his tail is injured. It’s not a bite, it looks like it’s been stepped on. I suspect my mother in law but it kind of doesn’t matter how it happened, it was an accident regardless and I just have to deal with the fallout.

The vet gave him pain meds and anti inflammatory meds. We’ve been giving those as directed. The vet said the peeing might also be stress related. Then he started having diarrhea. He pooped all over the living room and on a bed again (with a person in the bed at the time). So we put him in a small room with a litter box, food and water so we could sleep last night without waking up to poop streaked all over the house. We called the vet and they said if it doesn’t go away in a few days to bring him in. Overnight, He had more diarrhea but in the litter box this time. He’s doing ok in there for now but this is not a long term solution.

THEN we found another cat outside last night. Cat D. He is very friendly, healthy, and no microchip. Male, neutered. He just set up camp on our back deck and meowed like he wanted to come in. So I brought him in and put him in a separate room away from all the other cats. We will try to find his owner, he seems like somebody’s pet. But if we can’t find his owner, I don’t know what we will do.

My husband is ready to re-home cat C (the kitten). I said we should wait a few days, but we can’t just keep him in the small room where he is. We also can’t have him pooping all over the house. If the peeing is related to stress then introducing cat D does not seem like a good idea.

We got feliway diffusers for upstairs and downstairs, hoping everybody calms down. We got orange scented odor spray to hopefully deter cat C from peeing on beds. But the situation is untenable. I have kids, a job, and I’m in school, and I cannot be dealing with cat drama all the time.

Sorry this is a novel, but it’s complicated and I don’t know what to do!