1. I think it was the actor's choice to leave at the time.
2. Yes, Diane had an "affair", it never became physical though. The man she had the "affair" with was Billy Hopwood, Andy's biological father.
3. Yes, Diane knew Robert. She came to the village in 1999 and Robert left in 2005. Jack and Diane was even married when Robert left Emmerdale. She has always had a close relationship with Andy, but if I remember correctly her relationship with Victoria became a little strained when her "affair" with Billy became known.
4. Yes, Annie Sugden is still alive and lives in Spain. I think Annie is supposed to be similar in age to Sheila Mercier who plays her, and she is 96.
5. As far as we know Joe Sugden didn't have any biological children. He had two step-children, Mark and Rachel, from his second marriage to Kate, but they're both dead. Mark was killed in the plane crash New Year's Eve 1993, and Rachel was murdered by her boyfriend Graham in 1999. Joe did spend a few years in France in the mid 1980s though, so there is an opening, should they want to, to introduce a previously unknown child. The Sugdens have been very bad at procreating, but very good at dying.....
6. I don't think anyone but Andy and Robert knows the truth about what happened when Max King died. Jack knew too of course, but he's gone.