Thanks for the report, I agree with your suggestions. IIRC, the "no such file or directory" is a kludge that comes from the inability of the FTP code to distinguish between different kinds of errors in RETR. In fact, Wget's FTP code is old somewhat crufty, although it works well in practice. The problems will probably be fixed at a later release, when I get around to revamping the backend implementations.
Patches that fix the problems in the current codebase are very likely to be accepted.