(above) This should not end the bike's existence.

The small screw is easily replaced. If the threads it goes into in the barrel adjuster are bolixed up, try running a tapping die into and out of it. The barrel adjuster outside threads, I think you can run an (external threading) die over the threads to fix it. That is the thread size where you need a locking nut, however the outside of the nut cannot be too large or it will press on the frame when the latch is closed, possibly bending that threaded part. Mine is close enough that I orient the nut with one of the wrench flats parallel with the frame, for better clearance.

If not above, that is the most common hinge latch type that Dahon makes, you should be able to find a complete replacement kit online. Try a local Dahon dealer first, next a place that repairs folding bikes but may not be a Dahon dealer, then look online.