I am so sorry that you had to let Monty go, it is such a difficult and painful decision to make
But I think that you did the absolute best that you could for her and I wouldn't have chosen surgery for her at her age either. You sacrificed your own feelings knowing the heartbreak you would face so that she wouldn't decline more and could have a painless and peaceful passage. I don't think it's superstitious or crazy at all that you saw the rainbow. I think you were meant to see it and was glad that you did so you know she's alright. These things aren't coincidental. I am sorry for the loss of your dear girl.