A lot of good stuff there. Yeah, I think getting some kind of user store should be first. I think it needs to be server side so that it works with all browsers. I know you don't care about IE, but...
Brief answers first though:
1. Yeah, would require multiple tabs. Right now it just redirects and does not do an open window, so it is not an easy change.
2. Not currently possible, but agree it would be quite useful.
3. I've thought about stuff like this for a while, and it is very like the blekko slashtags.
4. Still looking for a good user account store, but haven't given it much thought yet.