And could we return to the people who are supposedly at the Thalmor party during Diplomatic Immunity? I've only ever seen there: Idgrod, Erikur, Maven, the cute Thalmor from Markath, the drunken guy. What do you have to do to get more guests to that party?
Me too! Might have to dig deeper with the Creation Kit and see if I can figure this out, if nobody else already knows the answer.
There are a couple of factors involved. One, some people are put there as potential allies in your need to have someone cause a disturbance so you can slip out for your mission. While I've used the silly drunk guy almost every time, other characters can be asked to do it, with success depending on your current standing with them. Some can be perusaded, others will act without needing a speech check. Because of this, some characters simply won't be there if you've "burned that bridge" already and they are hostile to you before this part of the quest. (Interesting note - if Ondolemar is there, using him for the distraction will cause him to be your enemy later after he realizes you used him against his own people.)
The other issue is what else is going on in the world at the time. The stage of the Civil War and other questlines (DB and TG, specifically) affects who will be there and who will be tied up elsewhere.