Malte,
I know the OS X Finder client has issues with WebDAV.
That said there is a newer version of tDAV that has better client support, but needs signifigant work to integrate with OpenACS.
Regarding folder name, your client uses the filename instead of the displayname (no client really supports displayname).
Renaming a folder should work. It works from Windows clients and cadaver at least.
There isn't any consistent way to know what files or folders are hidden, sorry, client inconsistencies.
