Of course the Queen will go to the wedding. She didn't go to Charles and Camilla's civil bit before the ceremony in St. George's chapel but then apart from the witnesses neither did anyone else as it wasn't necessary.
Whether or not the Duke of Edinburgh will be well enough to attend in five months time remains to be seen but I'm sure if they are both in good health they will be there.
Presumably she attended Princess Anne's second wedding - can't remember. I think she did but she could because it was in the Church of Scotland, not Church of England of which she is Supreme Head. I expect the A of C will advise her.
Ann couldn't have married in the CofE in 1992 , so little choice. Charles and Camilla couldn't because they had affairs whilst married to first spouses. Now the Anglican Church accepts marriage of divorcees queenie can attend this wedding.
I don't think Princess Anne's second wedding was a big occasion. It was very quiet and not widely publicised. I seem to remember that it was in Crathie Church which is Balmoral's local kirk. If the Queen was there, the press didn't spot her - she doesn't appear in any of the (few) photographs taken at the time.