1: yes and no, you can make 2 Xbee use the same channel if they talk to other Xbee (fe: you Xb1 talk to Xb14 and Xb4 talk to Xb11 and they are directly connected).
2: yes , this is the anti-collision algorithm but take care because if you don't see an Xbee that send, you will send anyway and maybe the receiver won't see it because he is already reading someone else.
3: yes, you can use AT command to change the used channel but 2Xbee must be on the same channel to send AND receive so you must change at the same time.
4: yes and no, there is a buffer so if he is receiving from different Xbee, the data will be send to this buffer but when the buffer is full, the data is lost. Also you can't say which Xbee send data in AT mode, you must use API mode to know additional information
5:no, but you can do an algorithm that switch channel than send
6: Yes is you use API mode, it depend on the type of frame you send (more information at http://ftp1.digi.com/support/utilities/digi_apiframes2.htm)
but in AT mode you only receive what you send so no additional data
Hope i help you