Keep things simpatico....where did the "church" thing come from? No direct bearing on MIL's good wishes.
How about this:
"Thanks MILName, your good wishes are appreciated. Best to everyone and you too! Lefty"
You need to remember that your W most likely will see these messages on MIL's FB. It is a permanent record and you can't take things back once it is put out there on FB.
