1. Max, sometimes XSI
2. Yes! Though, as already mentioned, there are a couple of competitors for Max (MaxScriptManager, which i've used in the past; and MaxScript Navigator, which i've yet to try out)
3. Ummmm.. Norman's MSM is a pretty good guide on what to implement, really. It has update notifications via scriptspot, for instance. I'd like it to be more customisable, the ability to group scripts, rename them and their entries in 'Main', possibly even bind keys to them from within the management tool. There's probably a million different things that'll come to mind later.
4. scriptspot and polycount
5. Some have builtin update notifications, such as your very own Outliner, which is very neat. Others you check regularly on scriptspot (i subscribe to their posts). Most code snippets posted here don't get updates on a regular basis, so..
6. Instead of doing it all over again, maybe you guys could team up with Norman3D? I think he has a lot on his plate, development on the Maxscriptmanager and on his MZP authoring system has been slowish, from what i gather. Also, cross-app functionality sounds like helluva complicated task..!