Are the three poppetts all the the same length?
Yes the poppet valves are the same length they just have different groves and step ups. Expect the black one is just all round and a smooth surface. They all will work with the idle adjustment screw. Just have to make sure the large spring is in place.
Are there any comments as to whether a bottom broken ring could cause spasmotic engine operation under no load conditions???
Yes broken ring or rings will cause what your problem is. I've found that with 2 strokes they will run with broken rings ,but just have problems like you've found.
I had one of my Twins run and run well it just didn't seem right. I thought it was a vacuum leak or a decompression valve problem. I found (when I pulled it apart to restore it) that one cylinder had two broken rings and a chunk out of the piston,but it still run ok.
So yes that will be your problem.
