Mixed signals to say the least. One day MIL is sending letter that uses fear, guilt, blackmail and then the next replies to text above.
So far you both have played it well. Above all do not get caught up in playing along with thinking you have to give an answer to every remark she wants a comment on. No telling but, she may have got wipped into a frenzy from a latest magazine, assembly or KH talk. Just reply to anything in a normal manner as you seem to do. As far as the 50th anniversay snub threat.....just tell her it is ridiculous to say that, and that all your family will be there.