I get the guilt trip. My mum is the same. I would just change the plans back and make up an excuse as to why it just had to be that day.
She may or may not be going away for the weekend. You ARE having a high tea. If she wants to be involved she will find a way or she can organise her own thing with you.
You said yourself that she doesn't understand that she can't reorganise someone elses plans. she won't understand if keep allowing her to. You will enjoy it better without her anyway. You will just feel guilty that she interfered with your IL's anyway if you went with the later time.
I love the train your mother idea.
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