1. The webrep can do nothing in how IE handles toolbars, if you don't have at least one toolbar enabled then it won't display.
I have IE9 (don't use it that often) on my other system win7 netbook, but I believe that was working last time I used IE9. However my win7 and IE9 are 32bit versions. I don't know if you are using the 64bit version of IE9 as I think that may well be an issue, though I can't recall where I saw that comment.
2. I don't use Chrome so I can't comment on that, but there doesn't appear to be a right click menu on the webrep page icon in Firefox 4. It just displays the standard toolbar options.
3. No, currently no Opera support.
4. I don't know any other similar rep tools that are selective, WOT shows for all categories. Currently there are no user settings for the webrep other than installed or uninstalled.