Great questions :)
The device has a standaby time of 16 hours, so it will depend on how it's being used. Remember that some areas where there is weak signal (or convergance of multiple cell towers) can drain battery life faster. The number of connected devices will also affect battery life.
Checking your data usage will be as usual in ClientZone. The device itself will also record some usage stats.
You should be able to use the device as a USB "dongle". It may require that you download firmware to your PC to allow you to do this.