For the throttle, i couldnt find much about flywithlua, but assuming reverse range occupies 20% of throttle range, and dataref goes from 0 to 1 could something like this work?
dataref ( "throttle", "sim/cockpit2/engine/actuators/throttle_ratio_all", "writable" )
do_every_frame ( [[
throttle=throttle*0.8 + 0.2
]] )