Well, here's part of the answer to your question.
When your iPad is logged into your UK account, any apps you've bought on the UK account will be updated if updates are available. Apps you purchased on the US account will not be updated.
If you then log out of your UK account and log in to your US account, apps you purchased on your US account will then be updated but apps you purchased on your UK account will not be updated.
I never ever go anywhere near iTunes - can't stand the program - so I'll have to leave that part of your question to someone else to answer.....
Tim