Regarding #2, there is also the possibility that if a civilization converted its light cone into computronium, we would not notice. Maybe because they used exotic matter as a substrate and that matter did not strongly interact with the rest of the universe. Or maybe they figured out how to "escape" this universe by creating a computer that can perform and infinite amount of computation without affecting its surroundings. This idea is explored in Greg Egan's Permutation City where the main plot involves creating a new pocket universe for simulated beings with access to an infinite amount of computation but can be bootstrapped on a normal computer.