[Skunkleton]

All network connected systems have a boundary where they become untrusted. With the exception of the GPS, this all seems like a securable, hackable, and supportable system architecture.

RE cellular modems, there are some things worth noting. First, they are a huge privacy hole. It doesn't matter if they run open or closed firmware, the real adversary is the cell provider's hardware. Second, interoperability is a real problem. There would be large consequences if enough improperly configured cellular modems ended up in the wild.

Edit: Another point here. Open firmware does guarantee security. It is unlikely that whatever distro you end up running on your pine phone will be nearly as secure as iOS or Android. Of course privacy is a different story.