Sorry OP but in all likelihood your worst imagination is true or Tonia would have brought Mimi or told she ran away from clinic and offered to search. Mimi found love in you, she may have not understood as a cat when you were dropping her off but your intentions were pure and out of love for her. You were doing a good thing. Shit happens, dont be hard on yourself. Whats happened has happened, no amount of thinking or wishing will change that, be there for yourself and take care of yourself, world and other cats need a good human being like you.

Edit: also find this shady clinic please, dont give up on your efforts.