I haven't read every word in this thread but I don't think anyone mentioned the most important feature of PCM over PPM. That is, the PCM receiver only needs to detect if a pulse is there or not. A pulse can be corrupted by noise and still be detected as a valid pulse. In a PPM system, noise injected in the rf link is passed on to the seros and the servos jitter. FM improved the rejection of rf noise over AM but it is still not as clean as PCM.
Jim O