Second, say a large economic force, such as EOS or a hostile political entity, bought enough ETH to violate the 1/3 stake assumption - are there any safeguards to protect against this unlikely but not impossible event?
If they get between 1/3 and 1/2, then we can still have some safety conditional on a synchrony assumption. Beyond 1/2, you would need subjective resolution (ie. an honest minority soft-forks the chain and the market chooses that fork).