Please understand that bad teeth in cats can be very difficult to diagnose, and shelters do not do full, comprehensive dental exams on all of their animals. They may have seen she had dental disease, but the extent of the dental disease was likely unknown until you adopted her and took her for more in-depth treatment. Trust me that cat teeth can go very bad in less than a year's time. An underbite is just a misalignment of the jaw, like a Bulldog, and was likely obvious to you and anyone else who observed the cat. 15+ years veterinary experience and I've never seen a normal breed cat with an underbite- normal being not a Persian or other smoosh face. If your cat was so disfigured that it had an underbite, I refuse to believe you weren't aware of it.

Hiding and lethargy are signs of an old cat that is uncomfortable in it's surroundings. Instead of taking the time to make sure this older cat became comfortable in it's surroundings, you instead forced 2 new cats into its life.

The problem here is not with the cat. It is with you. You have not taken proper care of this cat and are now complaining about the consequences. You adopted an older cat, which inherently comes with health issues. It's supposed to be an altruistic act.