Laur, I haven't read the entire thread, so I have missed some stuff but.........
First I second the second opinion with a sports vet.
Second, what you are going through happened to a friend of mine, amazing young dog that she had put a lot of training into.....bang problem (patella) and vet said no agility. My friend was inconsolable at first, which is understandable and the point where you are now. She finally took a big breath and consulted with a sports vet, and put her dog on a very careful training/conditioning routine.
Her goals to start competing in agility were sidelined for the time being, but with careful conditioning after the rest period, they started back at agility training.
Jump forward 2 yrs, dog is a rockstar in agility, competing, Q'ing/Titling and is a fast, talented dog. She is also not a small breed like yours is.............
Is her patella perfect? No. Can she run agility and not be lame? Yes. Does the dog know or care? No. Is she happy to be doing agility? Yes
Had a very nice lesson today with that dog, that 2 years ago was told she couldn't do agility...........yeah tell her that
And btw, anytime she has had problem with the patella was from rough housing with other dogs, never on the agility field.
Good luck.