You are not correct in that PPM(FM) moves the servos in 512 steps. PPM is pure analogue and the servo movement has an infinet number of steps as opposed to PCM that has 1024.

PPM would never switch the channel signals. The signal is sent with the channel signals after a wide coded start pulse. If it doesn't see this start pulse the other channel pulses will be ignored till it sees the whole sequence complete.

Everything I've read, and that's a lot, leads me to believe that PCM is more resistant to noise and interference so that's why sometimes glitches that were present with PPM disappear when changing to PCM.

I had a glitchy antenna once and the machine would go into failsafe, everything to hold except throttle to idle, and the glitch would disappear as the rpm dropped. It gave me a chance to save it when the system returned from fail safe.

This is my opinion only but I'd rather have a heli go into failsafe and drop than to have it glitch and go out of control who knows where.

Lots of people fly PPM with out a problem and others prefer PCM. I prefer PCM. You use what you fell comfortable with.

Good luck