You truly sound like a very loving, thoughtful owner. Realizing what’s best for both of you is a selfless act based on love. Some people keep a greyhound even when it isn’t in the greyhound’s best interest which I find troubling when the pup could thrive in a different environment.
A lot of people really judge others when they return their greyhound and don’t know all the facts. My belief is that if you are going to return a greyhound, for whatever reason, it’s in the best interest of the greyhound. Greyhounds adapt to home life very well after the track in most cases, and this life is completely different from their training/racing years. Luna will be fine eventually, and she is lucky to have you because you can prepare potential new owners about her personality and traits which may lead to a successful transition into her new home.