I can't imagine what you are going through, as Jimi is my first dog.  But my boyfriend's family dog, Tipper, was 17-18 years old (he was a rescue) when they took him into the vet to get a few rotten teeth pulled.  It was a routine procedure, but they severely broke his jaw.  They wanted to do the surgery but Tipper was in so much pain, and he was very old, so they decided to put him to sleep.
 
I am so sorry to hear about your situation.
No matter what you decide to do, just know that you have done such a wonderful thing by taking this baby in and loving her.
Good luck & sending hugs your way.. 
     
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 ♪ Jimi | Marshall | Hendrix ♫
"When the power of love overcomes the love of power, the world shall know peace."  - Hendrix