> separation of powers issues ... in the background

Under the Exceptions and Regulations Clause in Article III, every federal court exercises its power at the sufferance of Congress (apart from cases under SCOTUS's original jurisdiction). Securities-fraud cases brought by the SEC are creatures of federal statute, not (non-existent) federal common law (cf. Erie), and so the Seventh Amendment jury right arguably doesn't apply. Under the E&R Clause, Congress could authoritatively say so, overruling past SCOTUS ipse dixit extensions of the Seventh Amendment to analogous cases.