seat reservation not required. But would you recommend it?

You will also need to provide a travel date in advance, so that others can help you here.

Date 27/07

There is no such a btrain.

Coose between-part way, NightJet MUST be reserved-often explained here

OR change Düsseldorf/Köln to oevrbight in ICE=daytime seats only train

The latter _ its optional- can REServe both trains for 4€ via bahn.de-also explained here dozens of times

Reservation is required on the Nightjet to Innsbruck via Munich. 

Reservations possible via ÖBB tickets.oebb.at → one way ticket, discount Interrail/Eurail Globalpass. 

For day trains is it possible for the hole trip for 4€ via bahn.com with the seat only option. 

Do you mean the night train from Düsseldorf to Munich, departure around 23u30 ?

This is without mandatory .seat reservation