Update and conclusion to the story: just as I was about to tell Dolfi to stuff it and send the bag to Rimowa, the courier showed up at my house (unannounced of course) to grab it, so we hesitantly let them take it.
It was sent to a repair centre in the UK - and not to Germany - and was fixed and returned to us fairly promptly so happy ending, but overall the experience was really lacklustre:

0/10 for their communication ability. They keep sending the same template emails over and over again, they don't read or respond to your emails or always one step late. Missed about 4 collection dates. Sent the collection unanounced out of the blue. (Was an inch away from refusing it)

The UK repair shop did a good and prompt job but they weren't able to source original Rimowa parts so just got a generic stopper instead (which honestly is not really a big deal - they did call us to ask whether to proceed).