a) Only if you consider native iso-8859-1(15) “speakers” people a majority – every one else require these additional interfaces. I hate to go that way, but fact is that Internet Explorer still have these features in its RMB menu – they target more then just North America/Wetern Europe, and I believe they know their target market.
There are other RMB menu features I enjoy – and that I discussed above – that may not be used by a lot of people at all, maybe most of the people, but still – If they weren’t in the RMB menu then there would have been no way to get at them.

I don’t think that editing the RMB menu is a good choice – its far too complicated and most likely more complicated then the people, who use these features, can handle – hence you’d be effectivly removing them from the menu.
Possibly we can have a collapsing menu, similar to what Windows 2000 pioneered, where the less used (or more advanced) options are hidden behind a “collapsed” marker. When you hover or click the expander, the entire menu comes into view – while you can still clearly see which enteries will be hidden next time. The interface can also be intelligent and remember often clicked enteries and bring them out of the collapsed collection.

b) I though this is what Konqueror currently does ? play around with KParts and you can see how different options are added and removed from the toolbars as the views change. You can edit the toolbar and check the “merge” entry. If you add another toolbar and put the merge entry there (and remove it from everywhere else), you’d effectivly created your “tow toolbar approach”. I personally don’t like it – I think that having more then one toolbar really hurts usability and we shouldn’t go that way.