The FindMy network is already doing this at scale with location data. Offline messaging might be too niche of a use case for Apple but surely they’ve considered it.
One huge advantage of this (beyond better networking) would be that apps could use existing IP APIs which would help abstract away vendor/implementation specific problems and improve adoption.
Note that this doesn't necessarily depend on a tier 3 network so you could still accomplish your goals. The internet already has enough features to support partitions and local discovery.
You of course still have to have fixed administrative domains but in reality you always do. Someone takes initiative and sets up the group chat/gets their friends into it. I think if you're willing to mentally separate the network topology from the administrative topology this could be solved.
Of course that's really boring. It's just MDNS and ad-hoc Wifi plus some routing. Everything is pretty much already there (although iOS probably won't let you do it, as usual.)
https://support.apple.com/guide/security/find-my-security-se...
My interpretation here is that the location of a missing device can be passed around via Bluetooth mesh but that the last send must be sent via cellular or WiFi back to iCloud. So it’s P2P to relay getting location info back to final device with internet connection back to iCloud but the finder still need to connect to iCloud to view updated location.
“Find My works offline by sending out short range Bluetooth signals from the missing device that can be detected by other Apple devices in use nearby. Those nearby devices then relay the detected location of the missing device to iCloud so users can locate it in the Find My app—all while protecting the privacy and security of all the users involved.”