As I understood, there was a three position toggle switch that made the Rand actuator go all the way round either way for throttle control.
Meanwhile the rud. & ele. controls were flapping round neutral.
Raymond connected the switch to propo Ch 3 for a 1000-1500-2000 channel pulse.
What was needed: a propo to propo servo board that made teh servo slowly going open or close, depending on the input being 1000 or 2000.
Or better >1700 : open and < 1300 : close. between these values, servo stays put.
I nicked Phil's AtTiny elevon mixer for the in and output lay out.
Next I added some stuff for making it work, and it did.

Took me a while

Of course, when there is no interrupt, the output won't be running either...

Never mind, failsave is up to the 2,4 receiver then....

Testing with a pot instead of a switch, you can clearly see the 1300-1700 neutral 'window':
Edit, I've moved the MP4 file to YouTube - Mike_K