All good input and possible causes to the high, warmed up idle. I'll start with the simple things (leaking boots, etc.) after double-checking the carb sync and work my way through as necessary.
On the advance unit sticking, I don't think that would be the cause on my bike since it advanced and retarded fine when timing with the light yesterday, but I'm not going to rule it out. It would indeed hold the idle up if it's sticking advanced. Easy enough to check with your method. That indeed should shake them loose if sticky.