Given her age, if I were in this situation, I'd lean towards treating symptoms (manage pain, give softer food that's higher in fat, etc.) & plan on euthanizing her once her quality of life is starting to worsen. I've not had experience with oral tumors specifically, but everything I've read about them is how likely they are to come back. The other issue is if they're in the jaw, removal is going to severely impact quality of life after surgery - it just doesn't seem worth it to give her a few more months, especially if much of that is spent in recovery from surgery. I would spoil and snuggle and enjoy her company for now & prepare to euthanize once it's necessary. I'm sorry you're dealing with this.