When Shrek, Fiona and the king and queen are sitting down to dinner, note that there are only 4 chairs - all occupied. Just after Fiona burps, the camera flicks to the Queen - no chair on her left. Next, you hear Donkey outside the room - when he comes in, he sits down on the chair on the Queen's left....
as you mentioned about the princess, lack of english accent may be the result that she never heard the accent growing up. My brothers, and I were all born in Chicago ILL. in the last 10 years my brothers have lived in Memphis TENN. and now talk with a southern drawl, so basicly this falls into "nature vs. nurture"