Perhaps past compacts have been enforceable by federal courts. Still, the uniqueness of the scenario I've proposed makes me doubt those precedents apply. Say that California mailed in its electoral votes for candidate 'A', in violation of a (years-ago) compact, but in accordance with (1) its popular vote; and (2) its current legislature. Would even the Supreme Court in such a case say: (1) discard those votes; and further (2) count them instead for candidate 'B' instead -- a candidate who may be incredibly unpopular in California? That's a tall order, assuming a past compact can permanently encumber a state's exercise of its constitutional duties.

Nationally-significant fraud does not require national conspiracy. In fact, I think the national coordinating bodies of the political parties are relatively unlikely to want to see fraud. They're professionals; they care about their party's 'brand'; they can stand losing in a fair fight. It's the grassroots that's often unhinged, overzealous, eager to win at any cost (esp. if they think the other side is cheating too). That's the fraud I fear, totalling up to tens of thousands of votes in the aggregate, without any national coordination.