[greggold]

Again you have some interesting stuff going on!

I read recently that the dynamic power mechanism may not be 'fully enabled' if you don't have channel 5 at 100% (the 'ARM' signal)

Dynamic Power uses the ARM state to determine what to do when telemetry packets are missed. If armed, then each missed telemetry packet after the first raise the power one level. If the connection is lost entirely, it will raise the power all the way up. If CH5 is always high, your TX will always run full power when no receiver is connected (when there is no telemetry received technically). If CH5 is always low, your TX will never raise power due to missing telemetry but will still raise power when the telemetry reports the signal is low.

You could also perhaps try a filter of some sort on the power input to the receiver (using a resistor-capacitor filter for example)

You mention that the Rx voltage telemetry shows no issues, however telemetry is a pretty coarse time base so it might be doing all sorts of transient stuff at a faster speed than telemetry can report.

I have not tried to modify Ch 5 in any way. At the suggestion of Andrew Newton, I know some fliers assign (in mixer) Ch5 to a switch that they use for throttle disarm, with 2 positions 100 and -100 (or 150 and -150). I test my planes have a failsafe by holding them in position, applying 1/3 throttle, and turning off the tx. So I assume that the throttle automatically disarms when signal is lost. I am critical of the documentation which makes a fuss how important it is to use ch 5 for arming and disarming, but not specifying the method to do this. 

If I go to Mixer, and select ch 5, and edit, there are many items to adjust:
The second item "source" I leave blank (---)
weight is 100%
I see trim is on
the eighth item is switch.

Do you set the second item "source" to a switch?