i believe it would have come with one, can't say i remember for sure seeing one, but didnt see brand new ones very often, mostly used ones.
return spring should take into effect if you lose power, with power, it is expected that the failsafe on the RX/TX will close the throttle.
if the servo is...
i personally havent used a zip tie around the HSN, usually spring wraps around the HSN itself.
to check to make sure it will work, while the truck is off, hold the servo to open the throttle fully, then let go. if it closes the throttle, quickly should be good, if it is slower or doesn't...
dont know if there is any better way than back, just need a spring with enough tension to overpower an un-powered servo to pull the carb slide closed or pull the linkage itself to close the carb, but you also dont want the spring to have too high tension where the carb is having to really fight...