[cynwoody]

But the iPad is not integrated into the flight controls in any way.

Not unless ...

[Swordmaker]

Not unless ...

One problem with that scenario. Avionics are not ever on the same network as the WIFI that passengers are allowed to access. Nor are they connected by a WIFI networking system. No need for a Firewall with worry about Firewall vulnerabilities if the hacker on the WIFI never encounters the firewall in the first place. Problem solved.