Welcome, and glad you're enjoying it!
1. It isn't appreciably different from solo; well, depending on how everyone plays. Most will take 9+ hours I would say.
2. I think the wiki has a lot of details on that, but basically just go to manage players in the in-game menu (escape), and turn on or off players. If you add players late they won't have a home planet or anything, but they will have a command station home core.
3. Check my sig, or the sticky in this forum.
4. There's not any particular limit, but what you're describing sounds... immense, if it takes that many words. We're always happy to have sugestions, but we'd have to be in an expansion development phase to even consider something that sizeable. Right now it's pretty much small tweaks and balance fixes, but we encourage submitting any ideas ou have while you are thinking of them because next time we go through a big development binge we always go back and look at the prior suggestions. Our next big push is likely Q1 2012.
Again, welcome!