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.
Congressional consent is not required for the National Popular Vote compact under prevailing U.S. Supreme Court rulings. However, because there would undoubtedly be time-consuming litigation about this aspect of the compact, National Popular Vote is working to introduce a bill in Congress for congressional consent.
The U.S. Constitution provides:
“No state shall, without the consent of Congress,… enter into any agreement or compact with another state….”
Although this language may seem straight forward, the U.S. Supreme Court has method, in 1893 and again in 1978, that the Compacts Clause can “not be read literally.” In deciding the 1978 case of U.S. Steel Corporation v. Multistate Tax Commission, the Court wrote:
“Read literally, the Compact Clause would require the States to obtain congressional approval before entering into any agreement among themselves, irrespective of form, subject, duration, or interest to the United States.
“The difficulties with such an interpretation were identified by Mr. Justice Field in his opinion for the Court in [the 1893 case] Virginia v. Tennessee. His conclusion [was] that the Clause could not be read literally [and this 1893 conclusion has been] approved in subsequent dicta.”
Specifically, the Court’s 1893 ruling in Virginia v. Tennessee stated:
“Looking at the clause in which the terms ‘compact’ or ‘agreement’ appear, it is evident that the prohibition is directed to the formation of any combination tending to the increase of political power in the states, which may encroach upon or interfere with the just supremacy of the United States.”
The state power involved in the National Popular Vote compact is specified in Article II, Section 1, Clause 2 the U.S. Constitution:
“Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors….”
In the 1892 case of McPherson v. Blacker (146 U.S. 1), the Court wrote:
“The appointment and mode of appointment of electors belong exclusively to the states under the constitution of the United States”
The National Popular Vote compact would not “encroach upon or interfere with the just supremacy of the United States” because there is simply no federal power—much less federal supremacy—in the area of awarding of electoral votes in the first place.