For places, is it significant that there are sometimes trailing slashes, sometimes not?
It's not significant; that's copy-and-paste sloppiness on my part. Plone automagically deals with either formulation. We could get pedantic and say that some of these are containers and therefore ought to have no trailing slash, but then there's the fact that implementation-wise a place object is a container too. But I don't think users need or want to have to think about these things this way.
So, I think canonical URLs for places and names and lists of same have no trailing slashes; however, our application will pleasantly handle trailing slashes.
Stay tuned for responses to Sebastian's other points anon ...