1.) yeah till the last German Station it´s reservation free :)
2.) i managed at quite easy via CFL (as the bus is operated by them) CFL is the national Rail of Luxembourg :)

Does that also mean that there's no reservation needed between Frankfurt and Saarbrücken when it's a TGV?

correct within Germany the trains can be used without any reservation The reservation is only needed if you travel further than saarbrücken to Forbach/Paris :)

@seewulf how much did you pay for the reservation of the bus Luxembourg-Saarbrücken? Flex price seem to be only 5€ so I wonder if it pays off to go to the ticket counter or if price is almost the same? 

I asked at the CFL ticket desk in Luxembourg and they told me that the fee is always €5, with or without a pass. 

To reach Luxembourg without extra fees (and on trains), travel via Trier (or Konz). 

Thanks for the answer - good to know.

yeah, that route I know just would like to try this route as well but in that case will do it without a pass another time

In fact, passes are not valid on the bus Luxembourg-Saarbrücken, as stated in the terms and conditions, and reservation is optional, although it looks like seats have to be available, no standees allowed.

I thought I had mentioned this before. In any case, the app is wrong again.