In your case the main problem is that you have 12 devices each using at least 60-70ma, so I'm not sure how well the backup power feature would really work unless you used thicker wire.
Each unit has a separate 22/4 cable, so total current in the ground lead is 120 ma. 22 gauge wire resistance is 16 ohm/1000 feet. So at 70 feet at 120 ma, that would be a voltage drop (or offset) of 0.14 volts. I think anything less than 0.60 volts is safe.
I need to find out about CAN transceivers. Thanks.