It is a shame that no one has answered you on your CMYK queries but it is a big subject.
The CMYK_tiff-2-PDF plugin does what it says, can be a batch process, the file(s) does not have to be open in Gimp. Converts a CMYK tiff to a CMYK PDF. Tried it, it does work.
Gimp works in RGB mode. Commercial printers usually want CMYK files, these fit in with their printing process. Gimp will open a CMYK image file but converts to RGB. To save a RGB as CMYK you need to look at the 'separate+' plugins http://registry.gimp.org/node/471
There are binaries for MS Windows, if using a Debian linux there are binaries in the gimp-registry-plugins package.
Nothing is ever straight forward, you also need to know about color profiles, look it up on google or wikipedia, also some free files here
There are other applications that do the RGB to CMYK conversion, usually Imagemagick on command line but recently Inkscape has some useful (beta) extensions.