Thanks for the questions!
Devices on the same network will hop together, so it is only different networks that need to avoid eachother. Flutter hops across 50 different channels, but there is a space of about 1300 channels between 902-928MHz. Different networks will arbitrarily grab a channel schedule from that block, and will hop in a different pattern. So the chances of interference between any two networks, even if you had 50 different networks at once, is pretty low.
The actual scheme for channel acquisition is still in development, but it will be generated by the system based on the network ID and a random seed. Haven't pinned down the details yet. We do have to configure the boards to do listen-before-talk for the european market, so no board will try to transmit if the airwave is already occupied (it will try again in 20 or 40 milliseconds). That will help avoid collisions.
As far as crypto - SHA-256 is to my knowledge still recommended for use, but we are looking into using ECC crypto instead. Now that the official Arduino Wifi module has been announced using the ECC chip we have been looking at, we feel a little more comfortable going that direction. This will be determined in a few weeks, after we have finished wireless testing and are ramping up production.