I just had a read through this thread and coincidentally something I posted last night may be helpful here: see this Libudev package
In particular, the udevadm monitor command will print out the real kernel uevents as they happen and you can use it to see if the uevent itself is being generated or not. Just start it up with a procd script very early in the boot process and pipe the output to a file.
If you use this in combination with the hotplug debug script @anon50098793 posted above, you'll get a detailed view of the events being generated and being caught by the hotplug handler that should help to debug a bit more transparently.