[Marky]

(1) Most mods on vBulletin are pretty easy to install and most are very easy to uninstall. There is a new system on vB called 'Products'. For some mods, you just upload a product file via the admincp, edit a template and your mod is installed.
Of course, as you mentioned, it varies from mod to mod, but most are pretty easy

(2) There is an automatic upgrade script that comes with every update. To do a full update, you just swap all the files with the new files and run the upgrade script. Of course any mods that require 'Source-code' edits, you must re-edit them, but some of the product mods stay

(3) I think I know what your getting out, and it's already built in, but you'd have to have your users use special usergroups and permissions.

Hope this helps

Marky