Older thread but good topic. I'm new to gas too. I have a DLE 30 and a Spektrum DX7s. There is only throttle return on failsafe. What I did was a combination of things mentioned above. Great planes mechanical switch, re soldered to N.C. as above, operated by throttle servo arm with dubro servo saver spring installed to allow servo arm movement beyond throttle stop position. This cuts power to optical kill switch. I had to bind the receiver with my throttle trim all the way down (enough to actuate the mechanical switch). Then set the idle trim after binding. I then set my throttle kill to work off trainer button (-20%) to kill the engine on the ground.

This set up allows me to keep the throttle stop and return spring on the DLE carb. Kill the engine conveniently with trainer switch at any time and kill the engine in case of signal loss. If the servo fails I'm out of luck, but I could kill it even if I lost the whole linkage.