I've only just seen this thread, but I just wanted to say that we faced a similar situation with our Teddy 2 years ago. He was only 6 years old and had osteo in his shoulder. We agonised over what to do, but eventually decided against amputation as he was a very large dog and already had ruptured tendons in his other front leg. We decided to spoil him rotten and let him go peacefully before the pain got too bad. I still sometimes wonder if we made the right choice, but I do derive some comfort from knowing that he didn't suffer too much. From his point of view, he had a lovely few weeks and then went gently to sleep for the last time.
You have my sympathy.