I have been flying electric, but recently removed Mega brushless and installed OS .15 into my Piper Cub. First flight great, but I am not sure I have the throttle set up correctly.

Full up channel 3 and the throttle is wide open.
Full down channel 3 and the throttle is fully closed.
To keep the plane flying and to avoid dead stick I used channel 3 trim to 80% up.
To kill the engine after landing I quickly lower the digital trim to 40% or so and this kills the engine.

The first question I was asked at the field was if the radio has a "kill switch" and as far as I can tell it does not.

Is this true? I am assuming a kill switch over-rides the trim and closes the throttle immediately.

I have studied the manual and it appears as though throttle cannot be assigned to one of the switches.

Any help appreciated.

Throttle kill is a feature. More advanced TX's have a dedicated momentary push button which in fact bypasses programmed parameters to move the throttle channel all the way down to effectively kill the engine. The Flash does not have this feature and it’s not plausible to program same to the best of my knowledge. Your method for an engine out is atypical and works fine...