Can a pendulum stabilizer for airplanes really work?
As you noted, a pendulum can only "measure" pitch and roll angles when the pivot point is not accelerating.  Except, of course, if the length of the pendulum is the same as the Earth's radius — in which case we have a Schuler Pendulum.

Ed Carsten's pendulum stabilizer attempts to use the same principle as an autopilot measuring roll and pitch angles, and then generating elevator and aileron commands to level the aircraft. But this cannot work when using a pendulum of practical length since the roll and pitch angle "measurements" are in fact combinations of attitude angles and horizontal accelerations.