> Usually, you still have the “server is selectively lying” problem; unless the users are talking to each other, how do the consensus rules help with this?
If you're submitting txs to a node that doesn't communicate to the mainnet (they're isolated from it) then any txs that go to it would be void. You could just use that Eth on the proper mainnet as it wouldn't be on the chain. If the node decided to then come onto the mainnet it's chain would be vetoed by the other nodes states and would fork back onto the main chain. Ethereal has Byzantine-Fault Tolerance up to 50% and you don't gain anything by running an isolated node to try trick people.
That's not the only way to lie. You could, for example, lie that a transaction that doesn't exist has gone through – say, in a cross-chain “currency exchange”. Or perform a double-spend attack. Or many other things, because the Byzantine fault tolerance doesn't apply in this case.
If you're submitting txs to a node that doesn't communicate to the mainnet (they're isolated from it) then any txs that go to it would be void. You could just use that Eth on the proper mainnet as it wouldn't be on the chain. If the node decided to then come onto the mainnet it's chain would be vetoed by the other nodes states and would fork back onto the main chain. Ethereal has Byzantine-Fault Tolerance up to 50% and you don't gain anything by running an isolated node to try trick people.