The palace spokesman I saw on TV this morning, dug her an even deeper hole. He made the point that had the wedding taken place inside Windsor Castle, as intended, no one would have known if the Queen had attended or not.

Stongly implying (to me anyway) - that she would not have attended under these circumstances either........

Security issues or not - I think the idea that the mother of the groom not caring about her son's marriage - is a non-starter. I think we are left with the third option - that she does not approve.