Make sure your conversation with them is always that you love their daughter, you don't want to divorce, you've always been faithful to her, and that you want to fight for the marriage. Puppy
But isn't that redundant? Don't they already know that?
I think you might be be wiser to share with them that the door will remain "open" to your marriage until the very end.... but in the meantime you are healing, working to create a great life for yourself and child, etc... tell them about the fun things you are doing with the kid, invite them to join when you have your son (go fun places together!!! Outings your wife will regret not being part of!!!! And tell them this is for the grandchild....) Make them a part of your family!!! Impress them with how positive you are, what a great dad you are, and display all the great 180's you are doing (not bragging, but just mentioning things here and there, or sharing photos of things...). So rather than preaching to their D what a bad thing she's doing, they will be more likely to say awesome things about you and what she's missing out on. Make yourself the son in law they LOVE to be around.
There is no arriving, ever. It is all a continual becoming.