I can see where that would be heartbreaking but as everyone else has said, you are doing a wonderful job making the best of a bad situation.

Does she get along with any of the gang - on just a one-to-one basis? I know that how cats are in a group can be different than how they are one on one. I'm asking because I'm wondering if there is one in your gang that could go out to visit her every now and then and give her company. But maybe she is truely an only-cat and doesn't want company.

You've made a beautiful enclosure for her - its far nicer than living in a cage in a shelter.