So, up to now that's the situation:

GERMANY
1. Up to BREMEN-GOTTINGEN-KASSEL the routes seem to be more or less the same and, so far, they are clear
2. South of KASSEL, I see two possibilities:
2.1 Via Ederauen Radweg+Oranier Route or
2.2 Via Fuldradweg and Vulkanradweg-Trail, which seems interesting but I cant't find it
... however I have to investigate and find out which is the better/shorter one

SWITZERLAND
1. OK, I'm investigating on the link suggested

In the meantime, thanks to everybody.
Corrado