I can give a partial answer on why the plan sharing sometimes works in one direction but not the other - as told to me a long while back by iFly support. When both devices are connected to the same router, sharing should work fine. BUT when one of those devices (in your example, the iPad) is also acting as a hotspot/router, the hotspot device only allows devices to talk upstream to IT; and it cannot talk downstream to other devices (app to app). Only network traffic coming from the internet (Google, Yahoo, etc.) is sent downstream. Thus the 740 can share a plan upstream to the iDevice, but the iDevice cannot share a plan to the 740. Also, some home networks and nearly all public networks block device-to-device communication, to prevent hack attacks. This is a setting in the router software, and can usually be changed. Also, there's a setting in most routers to enable blocking of multicast messages, which is the method I thiink iFly devices use to communicate. So if multicast is blocked at the router, so will be iFly plan sharing. iFly cannot detect when this blocking occurs, so no error can be given.
To answer another part of your post as to how the sharing works, the iFly devices share plans by periodically broadcasting (multicasting) the plan "in the blind" for other devices to catch. The sending device does not know how many, if any, other iFly devices are listening, nor does it care. It just broadcasts like a radio station for anyone who wants to listen. It makes a broadcast when a plan change is made, and it periodically makes a broadcast in case another device has "tuned in" since the last broadcast.
I'm 98% sure the above is correct, or at least that's how I remember it.