Surely only giving guest Afternoon Tea after a wedding is very unusual for a high society wedding?
Like TryAnotherNickname I thought the upper class tradition was for light catering only after a wedding ceremony, before the bride donned her "going away" outfit and the happy couple set off on their honeymoon. I'm sure I've read in one of Deborah Devonshire's book her surprise at the first time she went to a wedding that involved a three course meal.
As for them moving from Harbottle & Lewis, people change lawyers all the time, and generally there's nothing sinister in it. It can sometimes be simply that you don't develop a good relationship because you don't gel. More unusual is for the lawyer to continue to act for their client - either because their fees are unpaid or their client has compromised the lawyer's ethics in some way. There is no suggestion that this has happened here though, so maybe Harry and Meghan met another lawyer they liked better, or fancied dealing with someone younger/more experienced/not so closely connected with Dad/with plusher offices - it could be anything.