Directly via CFL (Railway of Luxembourg) 

I suggest a more scenic Route with direct EC from Zürich to Koblenz and connect there to Luxembourg (if possibe actual many train lines are suspended due the flood ) 

Option B 

Via Basel - Strassbourg and Metz to Luxembourg :) 

I am most likely to take this option but on the DB website says that it is not required to reserve, even if I want to make a reservation it says that I cannot from that website, do you know where I can eventually reserve a seat?

It depends on which train connection exactly you are using. There are some reservation free with TER, other with TGV need a reservation. You should be able to make it via the InterRail app or if you are somewhere in France, then also at a local ticket office respectively ticket machine.

As @seewulf suggested, I’d also recommend the route via Koblenz… it is really scenic and worth it!

I am taking TER. Also do you know if IC trains operated by SNBC require reservation? The DB app says not just to check

No, IC trains in Belgium don’t require a reservation. Just hop on and travel ;-)

For TER you also don’t need any reservation on that route.