I have had cats disappear for more than a month, and show up all scruffy looking after I had given up hope. I have also had cats just disappear never to be seen again. It sounds like the stray kitten is depending on you now though, so keep it up. Cats do know their way home, even indoor ones. And she probably wasn't as terrified of the outdoors as you were thinking. Cats love all the smells and prey.