I'm just an interloper, eavesdropping on this thread, but I'm a little confused. I had a 'Peggy Clarke' as a yard tree that was adjacent to my bonsai benches for the purpose of yard decoration. I bought it specifically for that spot as a dwarf that wouldn't grow tall enough to shade my bonsai, but it got so big after about five years I cut it down. It was an aggressive grower and had no disease or bug problems. I would consider it a better than average Prunus, without problems, or bulletproof.
I hear all of you saying this is a problematic tree (in bonsai). I never tried to propagate it. I'm in USA 6b. The problem people seem to be in semi-tropical locales. I know that stone fruits are mostly northern trees. Is that the difference? Insufficient cold quiescent period/low enough temps during rest? This is intriguing.