What a great update, all things considered. Great sign he's still giving the kitties a hard time sometimes
I had a friend whose dog was definitely diagnosed with an aggressive form of cancer in his mouth - he was given a few months to live. I think it was about 7 years that she managed that cancer, until he died of old age liver failure. I'm not sure what kind of assumptions you can make about whether it is benign or not.
In any event, he has an amazing spirit and caretaker