Prunehead,
Regarding the use of AM for VHF comms, the reason is partly historical in that AM was around before FM but it also gives more reliable comms. This is because the AM reciever avoids the capture effect and continues to recieve the signal rather than lock onto the interference and lose the signal which is inherent in the design of a FM reciever.
An analogy is a conversation in a pub. When all is quiet there is no difference between AM and FM both clear, as the crowd comes in FM continues to give clear reception while AM becomes difficult. When the band starts FM only picks up the band while AM still continues to give comms with assistance from the stylized language used in aviation which has a high degree of redundancy.