He goes from trying not to smile while talking to Emma and John about John's boys, then as soon as Mrs E starts talking he has no trouble at all.

Poor Mrs E. She wants so much attention, and would probably get more attention from people if she weren't so annoying. But really, is Mrs E any less annoying than Mr Weston? They seem to me to be very much alike, but is our perception skewed by Emma - she is used to Mr W so he is OK, but she is not used to Mrs E so she is annoying. Or have a missed some important points about Mr W by joining the GR late?