After consulting the manufacturer I decided to fix the downtube cable adjuster issue by myself even though they offered to fix the issue if I sent the frame back on my own expense. I figured that mailing back and forth would just ruin the season for me, so I went ahead and bought a blind rivet package for 10€ at a local hardware store. Since the two holes were too close to each other I used a 4mm (or was it 5mm?) drill bit to increase the hole size manually about 1mm or so in the required direction. I was able to pop in the blind rivet and make the mend.
So far so good, I hope this will not affect the structural integrity of the downtube. I was hoping to get a waiver from the manufacturer before I started, but I think convenient language barrier got in the way. Anyways the manufacturer has promised to take a look at the issue once the season is over in case the problem persists.