1) Difficult one. It's semi-implied and I don't think we can be certain. His wife was only aware of the necklace and not any other business (hence the talk after the Christmas play when she gives the speech about him giving her the necklace and whether she should stick around or cut her losses and run).
2) Agreed with incitatus again - but maybe it's as if we're being told that they are together, but nothing can ever be quite the same given what happened.
3) I think she did. When she's apologising to David (Hugh Grant) she does admit "... and there was a fire" as in a spark of romance...
Great question though!