Max, sorry to hear of your loss. Good to know that you have a fighting spirit.

Anyway, like what Mark has mentioned, use a soft spring (maybe from a ball point pen of something), an elastic rubber band or an o ring as a mechanical fail safe. I use an o ring around my carb and wrap it on the slide carb.

What throttle servo are you using? The tension of the o ring / spring must be just enough to pull the servo arm back to neutral when power is cut off suddenly. Nothing more. If you use a weak throttle servo, you may have a problem. The servo may need a lot of torque to full open up the carb and overcome the spring / o ring. Making the spring / o ring softer, it would lack strength to pull the servo arm back to neutral position.

The way I test it is by throttling the servo at WOT and then unplug the receiver to simulate loss of power. The servo should come back to neutral position.