2
not sure what to say about your session question. that's a good one, heh.
nice job tho. if you want to separate each node into more of a site tho, you might want to check out my multisites patch in the XOOPS sf patch tracker. manually patch your files, if you use it, because the patch file isn't quite accurate (was manually edited after the patch file was created)
anyways, using symlinks and my mod, you could have multisites off of a single core install, but each site would have it's own configuration data, and could install it's own modules locally, as well as share modules globally. there's no security checks in place, so i wouldn't give free reign to someone, thinking that all the other sites would be safe.
if you want to go this route, pm me and i'll help ya along, and i'll also give you a copy of my in-action setup (based on 2.0.9 with further changes, whereas the patch is based on 2.0.7.3)