The quadratic hashing issue is a non-problem. Any miner that creates a block that takes inordinate time to validate will find himself bankrupted by other miners who continue hashing on the same parent to find a peer solved block. Such a peer solved block will validate well before the aberrant block, leading to the aberrant block being orphaned. 'Problem' solved. With the incentives as they exist today. Unchanged.
Nonsense. I'm not going to hope for some optimal-case scenarios with open attack vectors out there.
So in the case that multiple solution blocks to a block round are present, which one will be built atop? The valid block that validates promptly, the valid block that takes an inordinate amount of time to validate, or the invalid block that will never validate because it is counterfeit?