I agree about the DM - but I really do expect that several Middletons will be present - as that has become more normal over the years, as long as there is no medical reason to do so. Plus I am sure that family will want to assure themselves that the baby is indeed a Middleton and does not get switched at birth, just as happened in history. We are talking about the heir to Party Pieces, here! (and yes, folks - that is a joke, however lame).
It is a good question about a proxy for William. I wonder how the Lamaze classes have been going?