For anybody interested in game theory, I just realized that my reasoning in question 2 has an "unless." I wrote this:

I think it's fairly obvious that they can: a single individual is trivially better off using a policy which deterministically confirms the transactions with highest probabilities under the reference policy. If my reasoning is correct, this means that the situation where all nodes use the reference policy is not a Nash equilibrium.

The last sentence should say "... is not a Nash equilibrium unless...." Can anybody tell me what should come after the "unless?"