Denise, I'm sorry for all the pet troubles! I know how hard that is.
I have no experience with or advice about torn ACLs, but our Rocky (a husky/shephard mix) ruptured three disks in his spine when he was about 13. We elected to have him undergo surgery and therapy to regain his ability to walk, which was a risk and probably cost us enough that we could have taken two really, really nice European vacations. But I remember thinking at the time that it would be worth it if it got us another year with him. Well, he recovered beautifully and was with us for three years after that. He eventually developed a cancerous tumor on his leg and we gave him a peaceful goodbye.
So, long story, but the point is, Rocky's surgeon told us before we made the decision that he would have maybe a 60/40 chance of walking again if he had the surgery -- a long way from a sure thing. He was otherwise an active, hearty dog for his age, and I had just lost my mom (a decision to remove life support) so I couldn't even bear to decide anything that didn't give Rocky a chance for some more time. We spent a small fortune, yes, but I would make the same choice again today.
Do what is best for your Maggie and for you. I'll be sending good thoughts!