8 / 16 is the main powering. You can power the arduino directly with it. I would suggest a key on source, like the radio.. or something similar. You don't want it powered on all the time, it is roughly a 114mA draw.
The actuator itself will pull 10+A. I would relay it off the battery, with a fused 15A connection to the battery. You can vamp the lbb +12v line to power the relay if you need a switched power and all you have is inside the cab. The Shield cannot power the actuator.
The 4 modes, perf/cruise/brake/spare are all 10k pullups, so you just need a switch switched to ground to activate them. They can be exposed, it's entirely safe. Just may not be good if you get false triggers
The 5V line, is 1A fused directly on your Arduino itself. You can certainly power any sensors with it. The 3 adcs can be used to monitor their outputs as well. Definitely want to connect the ground back to #7 on the shield.
