Went with the original Thu/PI/Cologne Fri/Melia/Düsseldorf low/"high" backpacker-y combo in the end as Melia had their club ("level") room at 145eur for fri night. I'm guessing on Fridays Düsseldorf decamps to Cologne, I'll be a forellen going upstream. The 2 nights in Bonn idea is nice and it'll allow checking out their xmas market at night though.

Will book a free canx in Cologne for Thu in case the PI falls through.

Thanks for the Schreckenskammer recommendation! I'll stop OT-ing and will continue on the other threads.