[itimpi]

You cannot control what library Calibre portable starts in - the path is hard coded relative to the calibre-portable.exe program (over-riding the path you were trying to specify). If you want a different library you have to switch to it after the portable Calibre is running.

Maybe you would be better off using the calibre-portable.bat file from the Calibre resources folder for the basis of what you are trying to do? That is a little more effort to set up but does give you explicit control over placement of files.