You mentioned she is intimidated by the others and you have to feed her in a room by herself. Maybe the diarraha is stress related? You've given and done for Angel all the medical & home remedy stuff she just might be a kitty that needs to an only cat in a household.

I'd try her in a foster situation where she is only cat see what happens. It's worth a try, diarrahea cant get any worse from making the move. If it stops the diarraha that would be great and the problem is solved that she needs to be an only cat