In the first few decades of its work, the Supreme Court made extensive use of British court precedents.
The recent Supreme Court decisions hardly ever mention foreign decisions - and then almost invariably with regard to how treaties to which the US is a party are interpreted by other parties. Even then, the court is not "following" these foreign decisions, but merely using them as examples of legal analysis and interpretation of provisions that are the same in both countries.
FYI, MOST of the recent SCOTUS decisions as in Lawrence v. Texas were BASED on european law, to wit; "Kennedy, writing the majority opinion, referred approvingly to the British Parliament decriminalizing sodomy in 1967, the European Convention on Human Rights, and a 1981 European Court of Human Rights case."
'justice' (small 'j' intentional) Ginsburg recently gave a speech at some leftist lawyers guild where she also mentioned european law and court decisions as her reference for some of HER decisions. This is reprehensible and UNCONSTITUTIONAL.
And as to the separation of powers, I suggest you read the Constitution (of the US). The House of Representatives has the sole authority to IMPEACH judges - ALL federal judges, including those on the Supreme Court. And in case you forgot there's supposed to be three co-equal branches of government. The judicial branch is not above the rest and they are NOT supposed to make law, only interpret it, and our those would be our laws - not Gambia's or Lichtenstein's.