1.No.
2.yes.
3.Upto 1KM
4.There can be only one device on rs485 network transmitting at a time and all other devices will be listeners. Each listener will have an address and will pick up data relevant to itself. This is a convention to be implemented in the network by co-operation among the devices. There is no fixed protocol
5. Yes. But implement (4) above in software.

regards
k.ganeshan