@DavidSchwartz' answer, covers most of the question, but to address the last point:
If difficulty were perpetually at minimum, would the probability of a 51% attack skyrocket?
A very low difficulty would increase the probability of a majority attack. This would not be a direct effect of the difficulty, but indirectly due to the time required for block-propagation. At a very low difficulty, the party with the highest hash-power would be able to churn out a lot of blocks quickly and could immediately start building on top of those again, while other miners would be hampered by the block propagation latency.