If you lose your on-board power, you're screwed no matter what. In this case, your fail safe isn't going to do anything, and the throttle return spring is not going to override the servo. This discussion has been had in here many times about the throttle return spring. It's only function is to return the throttle to idle in the event the carb's throttle arm becomes mechanically disconnected from the servo. This spring cannot override the servo. The servo will remain in the last position it was at when it was receiving a signal.