Hello gentlemen. I recently went through similar issues with two DLE20s and some rough running issues with an EME35.
My procedure now for a brand new engine is to disassemble the entire reed valve and block area, gently fine sand the mating surfaces, then use red hi temp silicone in a thin film on all mating surfaces and re-assemble. This is carefully done, including siliconing the bolt threads, just to be 100% sure there isn't an air leak in that area. Careful not to clog up any jets. Let sit for 12 hours before running.
It also seems like the smaller engines like to be well rich for that very first start, like 3 turns out on both needles for an easy first start. Once running for a few minutes the needles can be leaned to normal.
To make sure everything forward of the reed valve works we would inject gas into the plug hole onto the piston, screw in the plug and run the starter. If it fired the engine itself is good.
Could be some sort of air leak. Maybe try wetting suspected areas with soap water and dry run the starter to see if any bubbles appear.
You've tried almost everything else. Good luck.