Okay now lets take it one question at a time
1) FM/PCM-The only difference is that PCM transmits a form of morse code on an FM carrier frequency while FM transmits like an FM radio station. The PCM is more resistant to interferance than straight up FM.
2) Channels-The 9C can have the channel changed by inserting a different frequency module into the transmitter and installing a matching channel crystal in the receiver. Another option is to buy the more expensive synthesized module and receiver, needing only channel selector controls to be reset to the new channel.
3) At the field, channels are not exclusive to one person, so many people may be on the same channel. It comes down to waiting for your turn or changing your radio to a channel not presently used by another flier. If it's not your turn, you must leave your radio off. Otherwise, your radio can cause interferance that can cause another fliers plane to crash. Each club/field has proceedures to prevent interferance from other flier's radios, which are normally strictly enforced.
Hope that answers all your questions