The PATHNAME serialization and ANSI-compatible namestrings issues should be solved as of [svn r12810][1].
1. A PATHNAME with no namestring now has a non-printable representation (i.e. "#<PATHNAME (no namestring) …").
2. The old, non-ANSI compatible #P(:host "host" :device nil …) output has been removed.
3. A PATHNAME namestring only contains '/' characters as directory separators. Any '' passed in are converted.
4. UNC pathnames can be constructed via '\server\mount\path' but have a non-printable namestring representations.
Hopefully this puts these issues to rest. Any other outstanding problems related to this issues should be reported as bugs.
[1]: http://trac.common-lisp.net/armedbear/changeset/12810