1: I was afraid the necessary libraries for xna would be bulky, I don't know how bulky though.
2: What needs to be synced with remote server?
3: Not having a campaign might be a small problem for the campaign.
4/6: I guess it would be possible just to launch ZK lobby for multiplayer.
5: Actually, my plan for account was to use .net browser to view a modified version of the account page that contained no external links (eg. it would keep unlock and commanders, but not have forum, nav-bar, etc.)
In-engine menu would be cool, and allow dynamic back-grounds with less worry of resolution than a video.
I might be able to replicate most of the functionality I was planning on having in Python with pyGame libraries so it would be cross-platform with no extra effort.