Remove system default / third-party app 'service' items from OS X context menus?
My own experience (on 10.9) is that the service uses the default search engine selected for the default browser, so right now my own "Search with…" says "Search DuckDuckGo for…" and will open in Firefox.



This doesn't answer your other questions, but it may mean you don't need to bother. I hope it helps. I've an old system running 10.6 so I'll give that a look when I get a chance and see if the same can be done there.