In my experience (with animals generally) the Jekyll/Hyde syndrome is most often driven by pain, and sometimes fear. That seems to fit Sissy perfectly. You're right - in a busy shelter it's very possible that she wasn't handled enough at a young enough age. As I'm sure you know, the first 12 weeks are crucial - anything that isn't instilled in that window will always be much harder for the animal to accept. A rush of new rescues in the first 6 weeks the shelter had her could be all it would take for Sissy's socialisation to have been inadequate...in part leading to all the problems she has now. This is why a lot of shelters prefer to foster kittens out so they can learn about normal home life when they're at their most receptive.
I'm well used to the laugh or cry scenario - sometimes even both at once. Just remember there is an end in sight. You've hit rock bottom with her, and it sounds like you're on the upswing (albeit still pretty low right now). You've been amazing to this little cat. It's blindingly obvious that you only want what's best for her (as well as the rest of your fur family). I really hope you get some answers when this is over.
I'm well used to the laugh or cry scenario - sometimes even both at once. Just remember there is an end in sight. You've hit rock bottom with her, and it sounds like you're on the upswing (albeit still pretty low right now). You've been amazing to this little cat. It's blindingly obvious that you only want what's best for her (as well as the rest of your fur family). I really hope you get some answers when this is over.