What are your exact travel dates? Some reservations might not have opened for autumn.
The routes seems perfectly possible, but remember you’ll only have 2 inbound/outbound travel days (home country days), in which you can use your pass in Germany. For now I count three days in which you travel in Germany.
This is easily solved by or staying in Strasbourg when coming from Barcelona, or buying a normal (non-interrail) ticket Strasbourg-Karlsruhe to save your last home country travel day for the long stretch Karsruhe-Kiel (whatever works the best out for you).
Raileurope.com is indeed a good place to reserve your seats, as is DB or ÖBB for domestic trains in Germany.
The dates are:
- october: Kiel - Mulhouse
- october: Mulhouse - Barcelona
- october: Barcelona - somewhere
- ocotber: somewhere - Kiel
Perhaps we use the way over Mulhouse again on our journey home. The costs for the reservation are much chaeper than a trip over Paris. In Karlsruhe we would like to visit friends. But that is noch so important….
The reservation for Kiel - Basel I will do about the bahn.de website.
Unfortunately Rail Europe introduced a 7,45€ booking fee as of today – but apparently only once per booking, so if you book a bunch of reservations at once, especially for 3 people, you should still end up saving money compared to the Interrail reservation service.