Gosh, IBD, I would love her...but with the two cats I already have (both territorial) and the three dogs

there is just no way. But, I understand the position you are in. Cats have definite personalities, and their dispositions run deep - especially if she is older than you thought. The fact that she was already spayed lends credence to her being an older cat...and either someone's pet, or a repeat kennel cat. And, unlike dogs who eventually figure out where they are in your family (aka "pack"), cats don't.
BTW, I had a cat that came to us as a stray - when we took her to be spayed, they too shaved her and ACTUALLY WENT IN...then discovered there was nothing to take out. They shaved further and finally found the scar on the inside of her leg.

The only explanation they could come up with is that it was done as a
very young kitten, and as she grew, the scar shifted.

I still had to pay, though, as she was under anesthesia, further prepped, took time, etc.
Anyway, do what is best for you and your family, and know that you did what you could.