If the Supreme Court can't refuse to enforce a law, then what is its check on the other branches? A recommendation that they change the law??

What kind of decision should the Supreme Court be able to make in a case where a citizen or a state sues the US government for having an unconstitutional law (a "controvers[y] to which the united states shall be a party)? In any other case, a court can require action by the offending party to rectify the offense. This generic power is not mentioned in the US Constitution, because it was part of the legal system of the late 18th Century, all of which was presumed to continue past the change in the nation's charter.

The Supreme Court is to have "appellate jurisdiction ... as to law ... with such exceptions ... as the Congres shall make."? This implies that all (previously existing) judicial powers continue to exist until revoked by law.

Those previously existing powers consist of every normal practice of courts operating in the states at the time. The Constitution requires trail by jury, but doesn't say how a jury has to vote to convict someone (unanimous or majority). It doesn't mention that a warrant is needed to conduct a search or seizure of property (though they are mentioned near each other in Amdt. 4). It mentions habeas corpus but doesn' t define it (despite it's history of mission creep) because its function was meant to be whatever it was at the time. The idea of not enforcing habeas corpus is specifically provided for.

Judicial process evolved over a long period in England and the colonies, and it improved without revolutions. The founders were not concerned with setting judicial procedures in stone, except to forbid those actions that had been wrongly committed by Great Britain before our independence. Judicial review existed then. It didn't apply to British law, but only because there is no higher authority in England--their constitution is merely law that they perceive as part of the character of their government and society; it doesn't supersede (lit. "sit above") other laws, and therefore doesn't imply an authority of the courts to override it.