The interstate State Compact, if it ever was implemented by states representing 270 or more electors would almost certainly be tested in court. Until it is tested in the Supreme court, nobody can authoritatively say if it is constitutional or not,
The arguments would centre around whether the constitution should be read on the "plain meaning of the constitution", this would be a "textualist approach", or whether we should consider the implied intention of the constitution (an originalist approach).
The textualist looks to the words written and argues that the constitution allows states' legislatures to appoint electors in any manner, without restriction. The plain meaning of the Constitution allows for the interstate compact.
The originalist would say "If we look at the structure of the union, it is clear that the intent was that the President would not be elected by the national popular vote. This idea was considered and rejected." So this would indeed be a "subversion" of the intent of the constitution.
The notion here is that the constitution delegates power to the states to choose electors, but as with other delegated rights (such as the right to free speech) that right is not unlimited (there is no right to shout "fire" in a crowded theatre) but defined by norms and historical precedent. By this analysis, the interpretation of the phrase "in such Manner as the Legislature thereof may direct" implicitly has a parenthesis "provided you don't fundamentally change the role of elector". An originalist view would not follow the plain text if they felt that my making the selection of electors dependent on the results of elections in other states would be a fundamental change and not sanctioned by the intent of the authors of the Constitution.
Ultimately this change can have political consequences, and so how you interpret the constitution is likely to be coloured by your political viewpoint.
And with the US supreme court being a political body, with appointees made as much on their political leanings as on their legal expertise, the decision in the supreme court could depend on political factors and not merely a plain reading of the text.
The US constitution has no procedure for the election of a president to be overturned. The electors send their votes to the President of the Senate, and the senate count and declare the winner. That person becomes President. If the senate consider the votes to be invalid then we are in deep constitutional doodoo.
If however the Supreme court decides that notwithstanding the lack of any procedure in the constitution, the constitution requires that the election should be overturned, then that would be their judgement.