Unfortunately both of your points might be effectively circumvented without violating the Constitution. Regarding your second point, as per Article II, "Each State shall appoint, in such manner as the Legislature may therof direct, a Number of Electors...". Hence there is nothing which prevents each state legislature from appointing Electors who will vote only for the candidate who has won a majority of the national popular vote.
Your first point is stronger, but it still only requires that a majority of Congress consent to the compact. Whereas a Constitutional Amendment would require a 2/3 majority of both houses of Congress plus ratification by 3/4 of the states. As long as Republicans either control the House or have enough votes to filibuster in the Senate, then Congress is unlikely to approve such a compact. (And it's possible that only a majority of the Senate would be needed if they tried to insert it into the budget reconciliation process.) But as soon as Democrats gained total control of both houses they could approve the compact. (Although this won't necessarily break down along strictly partisan lines. Some Democratic Senators from small states could find it politically impossible to vote in favor of the compact.)
So the NPV movement is actually a very clever end-run around the otherwise impossible task of getting 2/3 of the Senate plus enough small states to ratify a Constitutional Amendment for electing Presidents by popular vote. There's a reasonable probability that eventually enough states will pass the measure to total 270 electoral votes. I certainly expect California's Democrat-dominated legislature to re-pass it, and this time a Democrat governor is likely to sign it.