@kitten hmm, the schedule may vary depending on dates but bahn.expert is suggesting the option of Barcelona -[AVE]> Lyon -[TGV]> Bruxelles -[ICE]> Köln -> Koblenz, which could even be cheaper for Interrail than the route involving the Paris–Köln Eurostar...
@kitten hm, I suppose you could take the night train up to Köln, arriving 6:08, and then take one of the frequent IC/RE connections to Koblenz? slightly more sleep, and you could go sightseeing in Köln for a bit or something