Yea, it's really hard to tell from the pics, but if it was starting up real quick, and you could feel some good resistance trying to spin the crank, it's probably not horribly low on compression.
I will go blow into one of my stockers right now, but I can already say that if you were feeling significant resistance trying to blow through the exhaust, even with the cap off, that seems like a significant issue.
All the excess soot into your combustion chamber, top of the piston, head, cylinder walls, is likely due to not being able to fully empty the cylinder of exhaust gases, or a bunch of excess exhaust gasses being reflected back into the cylinder due to the restriction in the exhaust.
This exhaust gas, having not enough flow to make it out the exhaust ends up diluting the next intake charge so badly that it won't fire (4-stroking) until the exhaust finally catches up and looses enough pressure to allow a few strokes of good 2-stroking, then back to exhaust gas diluted 4-stroking.
This explains why the condition is always persistent, but come and goes as your riding through different throttle positions. Changing carburetion changes WHEN and WHERE it occurs, but won't ever fix it, like you have experienced.
I recommend attempting a run with the exhaust off completely, port open, and see if that corrects most if not all of the 4-stroking. You wouldn't want to leave it like that, but there is nothing wrong with it, many 2 stroke chainsaw engines came from the factory with no exhaust of any kind. The engine won't care, in fact it will like it a lot better then a plug in the exhaust port, I can guarantee that.
If you want I can mail you one of my old stock exhausts. All I use them for anymore is as scrap tube for random projects... I'm pretty sure I have one left out there in one piece.
