I also believe that Meghan is far less at fault than Harry. But the big lie I can never get past is one of hers . . . the wedding one.
I think you have to be a) old and b) know about how the Church of England works to appreciate how bad it was.
Meghan is a divorcee. The late Queen was very anti divorce and very straight laced. Princess Anne had to be married for the second time in Scotland because marriage of divorcees in church was not permitted in England. The Queen did not attend Charles and Camilla's actual wedding at the register office and although she attended the blessing and gave a warm speech at the reception, reportedly Camilla was furious because the Queen did not speak one word to her the entire day. The Queen organised a birthday party for Princess Margaret but refused to invite Roddy Llewellyn due to her disapproval of their relationship.
I appreciate none of this puts the late Queen in the greatest of lights, but these were not unusual views for her generation and social class. And of course the Queen's own uncle had to abdicate in order to marry a divorcee. It would have been a big step for the Queen to have agreed to Harry being married both in Church, at Windsor, and by the Archbishop of Canterbury (it would have helped that Harry was not the divorced party of course.)
I got married in a church for the second time in the late 1990s. It was very unusual to be able to do this - our vicar casually mentioned that we were lucky living where we did as he was the only vicar in the area who would marry divorcees. I doubt it was still as big an issue countrywide in 2016 but the Archbishop would still have given the matter a lot of thought, and would have had to believe that the divorcee he was marrying was not someone who took marriage lightly.
To have Meghan come out and say that he had actually married them in their garden three days before was dreadful. There are rules and laws concerning marriage and when/where it takes place. Meghan was claiming that the highest-ranking priest in the land married two people in their back garden with no witnesses and no paperwork, in other words that he broke the law. He was forced to issue a statement denying it. Had he been a lower ranking priest, a common-or-garden vicar, he would almost certainly have been hauled up in front of his boss and asked to explain himself. Luckily for him, his boss was HM Queen! Their next conversation probably involved her apologising to him.
As someone has mentioned upthread, the wedding rehearsal does feel lovely and just about everyone comments that they wish they were getting married at that point and stuff the big day and all the guests. But Meghan made it very clear that they were actually married before the wedding day. Huge lie and so unkind to the Archbishop who I expect had done a lot to try to make them happy and give them the perfect day.