I registered here after reading your review on the Dualtron Thunder and found it pretty much spot on.
I just wanted to offer your readers who might also have been experiencing it to try the fix we have done in my country (Philippines) where we have a thriving community of DT owners (we have about 150 so far).
The Squeaking noise seems to emanate from the stem or handlebar but you are partially right about the 2 bolts near the folding mechanism. Loosening it does affect the squeak but it's not a viable solution. The real culprit is the big bolt that acts as an axle for the folding mechanism. What you need to do to loosen it a bit and tighten it down again while spraying some WD40 on it. This has eliminated that irritating squeak.
Then also put WD-40 on the Quick Release Clamp as you push it down into position and lock it down.
Thanks and I hope it helps.