(above, post #6) I agree that the misalignment is clear, just like on my bike.

The key, if it's related to the handlepost base, is whether the base sits flat on the top of the headset, all the way around, or any gaps favoring the direction of post tilt? If no gaps, than either the post above is out of alignment, or at the hinge interface, or the handlebar clamp, and if none of the above, it's the bike frame. Let us assume the latter: I don't think you could permanently twist the main beam without a LOT of force, unless the twist is all at the main beam hinge; look for signs of mismatch in the two hinge halves as it closes. If not, then I think it's an error in welding, the parts not all fixtured perfectly as the frame is welded. There is also the possibility of warping during welding as the part cools, this is a problem with many parts, but that is usually worked out in pre-production samples by changing the welding order, direction, tacking everything and then finish welds, etc. So I doubt that weld-warping is an issue, with huge experience Dahon has with those design welds for over 20 years on similar frames. So my guess is fixturing error in welding. Good statistical process control gauges, with every part measured in a fixture or go/no-go gaged, would have detected this defect instantly. But volume manufacturers usually gage first and last parts on a shift, and all the others not.

Whatever you prove as the cause of the tilt, that's a manufacturing defect, not wear and tear. And I think you are well within warranty. The issue is proving it conclusively; you don't by chance have access to a CMM (coordinate measuring machine)? On a CMM with the frame latched closed, I would contact the inside of the head tube bore to establish its axis, and do the same for the seat tube bore. And, Dahon not blowing you off, just because they can.

When contacting Dahon, besides the serial number, you'll want to send a photo of the frame hinge, as the hinge on your Launch (Visegrip I think), is different from my Launch (Jaws). Dahon is now in California USA, in case paperwork with your bike says Illinois. But you may need to go through whatever Dahon subsidiary sold you your bike. Or your dealer if you bought through a dealer.