Kemmler was a huge evil threat, said to have many truly dark allies. Some of which would be the type to be confined in the well. So it makes sense for the warden to be involved in the conflict.
Only if he was trying to break dark allies out of the jail or release powerful beings to consume in a darkhallow. That would lead to The Warden's involvement to prevent the occurrence either on or off the island. However, as others have alluded to Alfred is pretty powerless off the island. The Warden may have been involved in the take down of Kemmler as a white council wizard or in his official capacity, but Alfred would only be involved if the incident happened on DR.
I'm of the opinion that the dinner party was to get the Vampires and the Wouncil to become allies and take over the Outer Gates. We know she traveled the Ways extensively. That would suggest that she knew about the Outer Gates, and who was defending it. I think she saw the need for shock troops (that the Vampires could produce fairly quickly) and the need for Magic (which the Wouncil would bring to the table.)
I think Eb KNEW that the Vampires couldn't be trusted, and didn't want to be part of the scheme.
I'm not sold on the idea of a proposed plan for the WC and vampires to join forces to take the outer gates. So far we have not seen any benefit to protecting the Outer Gates other than having a Purpose, greater prestige, and the ability to drag the world into oblivion. Doesn't seem to be much gain for protecting the Outer Gates (unless you argue Nemesis was working a scheme here). There is also the issue of reproduction of troops as that Quantus brought up.
I do agree however that whatever the plan was it involved joining forces with the vampires and was enough to royally piss off Eb.