“Matrix gives you simple HTTP APIs and SDKs (iOS, Android, Web) to create chatrooms, direct chats and chat bots, complete with end-to-end encryption, file transfer, synchronised conversation history, formatted messages, read receipts and more.
Conversations are replicated over all the homeservers participating in them, meaning there are no single point of control or failure. You can reach any other user in the global Matrix ecosystem of over 40M users, even including those on other networks via bridges.“¹
Matrix “rooms”, including group chats, are (optionally) end to end encrypted. Only clients have the keys - but most clients support backing up the keys to the homeserver - encrypted by a passphrase or master key. The server has access to meta-data, but not to contents of encrypted chats.
You do not HAVE to federate with all the other Matrix homeservers. I suggest having a server at each field office - and whitelisting only those MAF approves. MAF personnel need only connect to their local field office to participate in conversations. The field offices synchronize via the Matrix protocol with other MAF offices and HQ. If connectivity is lost, homeservers synchronize conversations when connectivity is reestablished.
You can also allow federation with other homeservers as requested by families of MAF personnel. You can add rules to allow attempts to join Matrix “rooms” only when meeting MAF guidelines.
¹ See “Messaging” on https://matrix.org