How is it remote play? It is a full web client.

Turn off your phone and lets see how that "full web client" behaves.

E2E with multiple clients is clunky at best.

Why? should be so easy to just send it once per device.

That might work for incoming messages.

* How does your history get synced when you send a message?

* What if some of your devices are offline?

* What if you have two devices that are never online at the same time?

* What if a device is offline for a very long time? (No possibility of holding the message on a server for eventual transmission.)

* What if there's only one device currently on that receives a message and then is destroyed?

Marking a single device as the master and forcing all other clients to proxy through it removes a huge amount of complexity.