General Discussion

If Trump had picked up a few million more votes, so as to win the popular vote, there would still be the argument that he bested Clinton in the popular vote only because of foreign interference.

It's comparatively easy to write an amendment that would abolish the Electoral College and elect the President by popular vote. (Getting it ratified, of course, would not be at all easy.)

A new amendment to provide a rerun of the election if we have "a foreign country interfere in our election" would be much harder. What constitutes actionable interference, and who decides? If Clinton had won, would Trump supporters be able to get a do-over by showing that the Prime Minister of Freedonia had said in a speech that Trump was an irresponsible nitwit, costing him support among Freedonian-American voters? Would the rerun election be available only if a jury (in what court?) concluded that the foreign interference had taken the form of violating some criminal statute? Would there be some requirement of showing that the effect of the interference met some minimum test of significance?

The only solution that readily occurs to me would be to establish a monarchy. The hereditary monarch would have no control over policy but would, at his or her discretion, be empowered to order a new election under such circumstances. The ratification of that amendment is, shall we say, unlikely.