I wonder how far you could mesh up the east coast, if you had mesh devices in every home, and what the round trip time over that range would look like if you're hitting a device every hundred feet or so.

I don't think something like that is possible. Without a centrally accessible authority, you cannot efficiently route data. Which is important with how low bandwidth something like LoRa is (and even Wifi may not have enough range/bandwidth). I don't think mesh can scale up like that.

Something like Meshtastic (FOSS mesh messaging project which uses the same LoRa wireless as Amazon's Sidewalk) blindly broadcasts, and any node that picks up the message repeats it. So with a big enough mesh, you'd use up all wireless bandwidth just repeating messages. Even if the recipient is right next to you.

If you could theoretically build a list of nodes, to efficiently route data by doing a shortest path, at some size the data needed to keep the node list up to date would eat up all the bandwidth. Because like messages in the previous method, every change in the node distance tree would have to propagate throughout the entire network.