Cats are territory oriented, and feral cats can take a long time to become accustomed to their new place. They have to get used to the new smells, sounds, people, etc. Cats do not instinctively know that people do not want to harm them. In fact, for rescues off the street, they may have good reason not to trust people, and their new caretakers have to put time and effort into helping kitty understand they are not a threat to her. The most important ingredient in making this a successful relationship is patience!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
When bringing a new cat home, especially a feral, it's always best to put them in just one room, make food, water available on one side, water on the other, and keep it dimly lit and as quiet as possible - though a radio playing soothing classical music is best. Because AC has the run of the house but stays upstairs, I wouldn't change this now.
But building up trust with a feral cat can take quite a long time. The best way to build that trust is simply to ignore the cat. The best advice for this couple is to tell them to think of this cat as a formerly abused child. Provide for her physical needs, and follow these steps to help create attachment, which only comes with trust.
Sit in the room with kitty - ignoring her. Read. Read out loud. Talk to her softly - but don't look at her. Sing. Knit. Do whatever they do. Let kitty get used to the new space and them. It's best to ignore kitty altogether, and make sure she has hidey-places. A box turned on its side with a cat bed in it. If she's under a bed, let her stay there.
If they're going to work in the yard or something, put on an old t-shirt or sweatshirt and get it good and sweaty. Place it near the food. This will help kitty begin to associate their smell with good things.
Leave some toys out (nothing with string of any kind!!!!!!!!!!) for kitty to play with. She will play when she wants to. But until SHE initiates contact, just ignore her. Evenutally curiosity will get the better of her. And the more they ignore her, the sooner it will be that she comes out from her hidey place to sniff at them, or "bump" them, or rub on their legs (marking them). It is important that they not get excited when this happens. It is still best to ignore kitty. But if she approaches the bed or chair where they are - just drop a hand down along side of the chair or something. Make no moves toward kitty at all. She'll come when she's ready. And if they do ever decide to reach out - with cats it is best to do this palm down (though it's really best if they don't even do that).
Right now she's scared. She has no idea what's happened to her. She's a kitten, and she's playful, and she feels safe enough in her space to mark her territory (which is what she's doing when she rubs her head on doors and corners) - but she doesn't trust people yet. And you have no idea what her past is like. Perhaps she was kicked by someone, or abused by neighborhood kids at some point. She doesn't know what love is, and wanting love or pets isn't something she even really knows exists.
Mistrust comes naturally to cats - although curiousity will eventually rule out, and that will then help build the road to trust.
And once a formally wild cat comes to trust you, there is nothing else like that bond. But it requires a lot of patience. Because she's a kitten, if they sit in whatever room she's in, and they generally provide a quiet atmosphere of ignoring her, she'll probably come around within a few weeks, but they should be prepared for it to be a few months. But the sooner they stop reaching out to her and instead simply make themselves available for her to reach out to them when she's ready, the sooner she'll be ready.
And while it is very important to play with cats - if they stop playing with the feather toy, that may help speed up the process (though there's no way to know). If they're just there in the room with her ignoring her, she'll eventually understand that they are not a threat to her. The more they ignore her, the less of a "threat" they are to her. She'll get comfortable enough to come over to investigate these big hulking things that call her, look at her, and try to touch her. And if she misses playing with the feather toy, and they keep ignoring her - her desire to play will help her approach them to find out who they are. But that won't happen until she is certain they are not a threat to her.
Also, tell them never to look her in the eyes. It's best to try to not look at her at all - that'll help her. But if/when they do, look at her forehead, not her eyes. And when she gets curious enough to check them out, let them know that "looking" at feral cats with your eyes closed can really help speed up the process of building that trust. I still do this with our cats, even though they're all just like any other pet cat now. They'll hop up on the bed and start towards me - and I'll look at their forehead, and slowly close my eyes and I'll hold them closed for 10 - 30 seconds, then slowly open them back up, looking over the cat instead of at the cat. They watch me while I pet them, and I "look" back at them with my eyes closed. You can tell it really makes them happy and comfortable.
When it comes to the two cats becoming friends - it may never happen. But they can help. When AC has become used to them, they do essentially the same thing between the cats. Take old wash cloths or rags of some kind, and wipe each kitty really well all over with it. Place the rag with AC's smell next to KT's food, and place the rag with KT's smell next to AC's food. They can also take rags with each cat's smell on it and do the same thing only place treats on it. The point is for each cat to begin to associate the other cat with good things.
But the cats getting along should be secondary to AC getting used to the idea that these people love her, and that's just something she doesn't understand yet.
Here are several very helpful articles on the subjects:http://www.thecatsite.com/Cats/Cat_C...a_New_Cat.htmlhttp://www.thecatsite.com/Cats/Cat_B...cing_Cats.html
It was so kind of these people to adopt her, and I'm so glad you're reaching out for help for her and them. As soon as they understand that it's not them, and that they should treat her like a special needs child because she had a scary uncertain life as a baby, I hope that will help them give her what she needs.
All of our cats are feral rescues, and believe me, when they come bumping us for love or meowing for play, or begging to be brushed, it's the most incredible feeling in the world. All but two of our cats hide whenever anyone but us comes into the home. That may never change. But when it's just the family, they're just amazing. I'm sure this couple will find the same thing with their little AC. But it can't be said enough - the most important ingredient is patience.