this is the workflow I currently use for my svn projects (I never use svn branches, and some projects are actively worked on by other people as well):
Unfortunately, svn's externals are quite flexible. I've run across a number of scripts that treat them as <path> <url>, but <url> <path> is also allowed. So I think some of the scripts are just broken in that regard.
To answer your second subquestion: no. 'git svn fetch' needs to operate on a subtree of Subversion's repo, but it needs to be something it can treat like a branch. The only thing map well into that paradigm is a directory (trunk/, for example). FWIW, Bazaar and Mercurial suffer here too. At the end of the day, Subversion is just a versioned file system, whereas Git has a first class concept of a branch. This is one of those mismatches getting in the way. :-(
Just a another note on the second subquestion. As far as I know subversion as of today also does not support svn externals for files, only folders. So I guess such entries in the properties would be invalid. Don't know if there is any tooling which handles this case.