Article VI:
“This Constitution . . . Laws, . . . Treaties . . . shall be the supreme Law of the Land; and the judges in every State shall be bound thereby . . . “
‘Nuff said.
Yes, but the schemer justices were very precise that their decisions would only be based on US law and precedent. But in reaching those decisions, they would examine foreign legal decisions.
Yes, it’s underhanded. But remember that these justices can only be removed by impeachment and conviction. So if they can muster 5 votes to undermine the constitution (like Roe v. Wade), they can.
But they can be checked by congress ahead of time, since they have the constitutional authority to determine how the judiciary conduct their business.
So if congress passes a law forbidding federal judges from doing this, it can to a great extent forbid them from acting like weasels in this case.