[assemble]

A pilot probably wouldn't know how all the systems work together. (They probably know what ACARS and ADS-B -do-, but not the implementation details.) You would want an avionics engineer--so someone who works at Honeywell, Rockwell-Collins, Garmin, etc.

If you can access the wiring in the aircraft, especially with the new fly-by-wire stuff, you could cause problems. None of the communications are encrypted (as far as I know), because it's pretty much assumed you can trust what's on the a/c buses. Also, redundancy helps get rid of erroneous data when it only shows up on one bus.

I don't know much about ACARS and ADS-B. While it wouldn't surprise me if there were problems, we also tend to notice rogue transmissions in our spectrum. Much more quickly than people realize.