Sneaking out of apartment

I'm sort of asking this for my older cousin and his wife. They recently moved from a rural area with a huge house and lots of land to a small apartment. They asked me to temporarily keep a few of their cats because the apartment has a pet limit and they can't have all their pets there.

So they want to try to sneak an extra cat in. My problem is that I don't think it's a good idea because I'm afraid the cat will escape from the apartment. Two of their cats that I have love being outside. When they were at the house, they would often come out of nowhere and zoom right out the door. This caused me to have problems capturing them when it was time to move.

I'm afraid that these cats would sneak out in an apartment. My cousin is in a wheelchair and even though he's careful when he goes out, there's no way to keep a determined cat from getting out. Even if they were to lock the cat in a room, there are other people that live there with them who aren't as careful. Locking the cat in a room everytime someone wants to go in or out will eventually become tiresome.

I'm afraid that if either of the two cats got out, they wouldn't come back. The oldest one is skittish and isn't used to living in the city. I know she's very attached to her people and would want to come back, I just don't know if she would find her way back, especially if she's frightened. The younger one is also skittish and is full of curiosity.

The cats aren't exactly happy with me. They don't like each other and are getting aggressive with each other. They don't have a lot of space because I'm keeping them separated from my cats for now, until I know they can get along without killing each other. The oldest cat does love me and I love her, but I can't give her as much attention as she would get if she lived with her owners. She's about 11 years old and as of right now it looks like they're going to be with me for about 5 more months, maybe longer.

What would you all do?

Thanks. I just feel so bad for both the oldest cat and my cousin. My cousin didn't want to move but his wife forced him (he's physically handicapped so she gives him no say).
He loves that cat and she loves him. He really misses her. He can't come over to my house because we can't get him up the steps into the house. I think it would be way too stressful on the cat to take her to visit him. I don't know if they'll ever see each other again. As long as my cousin has to rent (apartments or houses) there will be limits on pets. They will never be able to afford to buy a house. It's all so sad.