It really shouldn't be this difficult unless something is seriously wrong.
First even though 2 cycles are fairly simple there are several things that can go wrong. Even an leaky seal can shut you down on some engines.
First be patience. I have have at time felt taking a BFH to some engines at times but I just set them aside and come back later. 2 cycles can be frustrating at times to repair until you fully understand how they work.
First make sure you are using a known good plug. By this I mean one is working in another engine. The reason behind this is that even bad plug can be arcing very well outside the engine but be failing under compression. New plug doesn't always new good either as I have seen to bad right out the box. This can be anywhere from misses to complete failure.
Since this engine appears to have an auto decompressor setup verify that it is working as intended for if you don't have enough compression the fuel mix will not ignite.
This brings in the ignition timing. The flywheel is usually keep the ignition in time by the use of a shear key. If key is either partially or completely sheared then the ignition will be off hence will not be in time to ignite the fuel mix at the correct time.
As for the amount of normal compression for this the gentlemen on your side of the big pond should be able to advise you on what is normal. Most 2 cycles I work on here need at 100 psi when no decompression is involved, even when a decompressor is there they still need the 100 psi when it is disable.
Also one other thing that can cause problem other than carburetion would be either vacuum or pressure leak of the crankcase. I have even a blocked muffler port to shut the ignition of fuel as engine can draw in fresh air/fuel mix.
Now in reference to ether, personally I avoid it especially on 2 cycles instead I would just induce some fuel mix via the spark plug hole, install the plug, and see if it will pop (tries to start).
Edit: Our posts cross either other and I see you have narrow it down considerably so take the above post with a grain salt. Hopefully you didn't pull out to much hair.
