Don't worry about your wife's feelings on the mare. Your W made her choices, and now she has to live with them. Them's the breaks. If OM is so wonderful, why doesn't she ask him to help her keep the horses? She may try to blame you, but do not accept any responsibility.
I'll echo Ohio Mark's thoughts on your wife's statements about wanting to spend time with you. Be very careful. It would be very easy to get sucked back into letting her have her cake and eat it too. In my view, visiting the horses could very well be a ploy to guilt you into paying for their upkeep. Keep your power; you've come too far to give it up now.
I'm glad that you are spending Thanksgiving with people who are supportive of you. If you decide to send an email to the in-laws, a simple "Happy Thanksgiving" communicates your good wishes without giving anybody ammo to use against you at a later date.
