The Interstate Commerce clause seems to be available to SCOTUS whenever they want it.

An innocent person put to death despite evidence that would exonerate them is an issue of national-level concern on many levels.

14A says that "no person shall be deprived of life, liberty, or property, without due process of law".

I think it's reasonable to argue that "due process of law" should not, broadly speaking, involve executing people who are known to be innocent.

Well that's where Scalia's statement comes in - he argues that the state has no duty to avoid executing innocent people, only that all relevant procedures were followed. To use an example from history, if witchcraft was a crime, and the townspeople found someone guilty of witchcraft, they have no duty to accept a time traveller's defense that witchcraft is scientifically impossible and the person is therefore innocent. The "due process" was the trial and witch tests, not a modern scientific consideration of the facts. I don't know Scalia's actual opinion, but I would be surprised if he did not find these witch executions judicially justifiable. Presumably he would argue that the proper way to avoid burning witches at the stake would be to change the laws supporting such activities.