As a Brit married to an American, I understand where you're coming from - but it's a matter of what you're used to. I found dh's American manners really difficult to deal with at first.
But I soon got him trained.
Seriously, I have persuaded him to put his knife and fork together when he's finished eating - the untidiness really bugged me. Otherwise, his manners don't bother me as he's got good American manners. I think that bad manners stand out whatever the customs of the country you're in, and it's the bad manners that bother me, not the American manners.
Having said that, we are bringing our girls up with British table manners. It is interesting that you worry about your children being seen as rude in the USA if they use English manners. I see it differently - in the US, English manners are seen generally as quaint and curious. However, American manners in the UK can be seen as rude, for example eating with just a fork, which was an absolute no-no in my day and age. Having said that, things are so much more relaxed in the UK now that I really don't think it would be a problem for your children to have American manners there - just make sure you talk loudly with an American accent, then you'd be forgiven!
I do know that on many MBA courses in the US, students have to learn British table manners so that they can go to good restaurants in the UK and handle the manners thing. I don't think British MBA students have to study American manners though.
As for holidays and festivals, we love having dual nationality (plus German, as Dh is half German). We enjoy all the festivals (although I have to admit to not enjoying Halloween as like many Brits, I find it uncomfortable, but we do it because it is a tradition here). We do Bonfire night, make mince pies and Christmas pud at Christmas, eat pancakes on Shrove Tuesday, put out our shoes for Saint Nick like all German children, then also celebrate all the US holidays. I try to see it that our children get the best of all three worlds.
We're a few years into this English-American mix, and all I can say is - try to lighten up, not worry, and enjoy it. It is interesting, and it can be difficult, but it is also a lot of fun.
And cut your dh some slack when he bashes Americans - I find myself doing it, partly out of homesickness and displacement - we really are divided by a common language and if your dh is like me, he'll never really feel that this is his home. It's tough being away from home, especially if people don't like your table manners!