In a national popular vote, there'd be a lot more very-close elections -- see the marginal voter theorem. A two-party system is designed to create ties.

(Which also brings up another potential problem with ditching the EC -- it could encourage highly unstable multi-candidate elections, where fringe candidates aim to get elected or make a runoff with a small plurality. Unless our legislature and culture adapts to embrace a multi-party system, I don't think multiparty presidential elections would be a good mix-in.)

Fraud in swing states is much harder. By definition, sentiments in a swing state are nearly evenly divided -- both sides have a lot of power 'on the ground' where it matters. Plus, everyone's paying close attention.

The graduated allocation in close +/-1% situations is an interesting idea, and lessen the incentive for fraud somewhat. But, it can't help in a true national popular vote: ultimately, top vote getter, even if by 1 vote, wins.

People say they want rid of the EC, but I think they'd miss it if it went.