I would not allow her out on the balcony unless it is totally closed it. She may get frightened and jump out of instinct.
Yes, I have a cat who is terrified of the vacuum cleaner and I lock him in the bedroom when I vacuum. He now knows to run in there when he sees me getting it out, so all I have to do is close the door.
Awww, it would have been nice to keep the two of them together. You just never know with cats whether they will take to another one. I think if you prefer to have just one, then that's probably best for her too (and you). I get the feeling in the way you are describing her that she may have difficulty bonding with another cat.
I hate declawing
. Vets should stop doing this abuse to cats
.