If I run calibre.exe:
a} all my former settings are gone.
b) That is because if I run calibre.exe as suggested by you it will create (in my case) c:\Users\user\AppData\Roaming\calibre\
That is not the concept of portable and it is a bad advive as now I got all the "trash" in \AppData\Roaming that I wanted to avoid in the first place by using calibre as a portable version
So I don't recommend anybody interested in a portable concept doing this.
I'd really like to understand your decision of not changing the behaviour as calibre is a fantastic software. So I ask politely for your reasons of creating an default library folder and not be able to change that.
No offense just curious.