Cliff,

Your new shim installed, the top lip is above the clamp mechanism, should be fine. I position mine below the top of the clamp, better to hold it in place if needed when adjusting, and the lip diameter appears to be smaller than the seat tube diameter, so the clamp should not be squeezing on the rim alone, if done my way. I'd be curious if dahon defines which way is "correct".

Once adjusted, mark your seatpost position at the clamp with a sharpie marker, so you'll know if it starts to slip. If it does, remove any lubricant or rust-preventative from the inside diameter of the shim, and the outside of the seatpost; Having that between the shim and the frame, good idea, as dissimilar metals, to reduce corrosion. But between the shim and seatpost, that should be clean, for best grip.

The frame hinge, I adjust as tight as I can, while still being able to open it without too much difficulty. I cannot recall if you can just spin the adjuster, or whether you need to undo one end. I think you can spin that hex adjuster without taking apart one end, as I recall putting a nut on mine to lock the adjustment, and my adjustment has needed less adjustment after that.

Stem C-clip or circlip, also known as a Jesus Clip, because when it pops loose and shoots somewhere where you'll never find it, or over deep carpeting (never work on your bike over carpeting), you exclaim "JESUS!" Hardware store may have replacements. Local to me is an industrial screw supply that also sells retail, they have darned near EVERYTHING, they are a great resource. For example, I use cartridge brake pads, so easy to slide out the pads for cleaning without undoing the large pad attachment, which requires time-consuming alignment to reinstall. Well, these are road pads, so had a short bolt, I needed a longer bolt to use the concave/convex washers for V-brakes, they had just the right bolts. Also, the tiny screw that retains the pads, uses a tiny allen wrench, and that is blocked at the front fork, and I tried a ball-head allen wrench, but the size was so small, it was stripping in the screw head; I just replaced with hex-head screws, so can loosen or tighten from the side, and that store had the perfect screws for that as well; I just need to be careful when tightening, as the threads are quite small, and a wrench from the side has a lot more leverage, could easily strip the threads.