Thanks.

1. It looks like the fork is OK (no play when pulling the brake and pushing the bike back and forth): The small issue I'm having seems to be due to the clamp. I can feel it moving a bit when pushing on either end of the handlebar: https://gofile.io/d/QOpmtM

2. After adding a front derailleur, what solution is there to keep the chain from falling off the chairing when folding?

3. Does someone know who makes this low-rise rear rack?

4. Since Birdies are pretty scarce around here, but I assume they must be a bit more common in Germany… is there a good site that advertises second-hand bikes? Considering the price Birdies are sold in Europe today, I'm willing to take the train and grab one if the price is right.

So many questions :-p