I think that there is at least potential value in sharing lexicons. Of course, a down side is that it complicates set up.
This is where I say "YAGNI" and announce that I'll be happy to refactor the code if and when a real need is discovered.
On the subject of referencing lexicons by path rather than using direct references, I'm inclined to agree that direct references are better for simplicity and speed. It's easy enough to add a new index when you want to change a lexicon. (Well, there are some complications having to do with making sure that you get all the needed data into the new index...)
What was the use case for switching lexicons in the first place? I bet it was just someone idly playing around and noticing that it didn't work right... --Guido van Rossum (home page: http://www.python.org/~guido/)