Hi Dave,
The collar is used to assist in keeping the prop shaft from being pulled or pushed out of the motor. Many people think having one at the back end is sufficient, as this will keep the prop from pulling the shaft out of the motor. However, there are significant forces under braking/down lines, so using one at the front makes sense as well. It goes between the prop and the motor, tight against the front of the motor, and should fit freely in the hole in the firewall.

The motor is held in by the 4 screws through the firewall. A lot of guys are using the rear support as well, and that's all that holds the motor in place. I discarded the rear support after having issues with screws falling out, and eventually the wood splitting and fluttering about.

I use 2x450mah battery packs, and have more than enough for full days of flying with no worries (also Ed's regulators).