Isn't messing with TTLs a pretty obvious thing to do when communicating covertly? And wouldn't that kind of packet inspection only catch cases of TVs literally acting as packet-level routers, but not catch application-layer shenanigans like aggregating statistics in the exit node so that all the data going over your own network appears to be originating from the same device?

It's still implausible, but thoroughly verifying that your TV isn't spying on you isn't trivial.

There's no requirement that the mesh be IP-based or function as a layer-3 router...

Even if it were IP-bsaed, it would be extremely straightforward for one box to send a UDP message to another, and receive and retransmit at the application layer. No TTL nonsense, No multiple IPs showing up communicating upstream.

But if my TV is doing this, then by definition it's not acting as an exit node so how do I test it?