Opinions vary.
I prefer to go by actual history.
“Republican president Richard Nixon pardoned, commuted, or rescinded the convictions of 926 people. Among them are: Jimmy Hoffa – prominent labor union leader convicted of fraud and bribery (tax evasion) in 1964; sentence commuted (with conditions) on December 23, 1971.”
Some of those were state level crimes.
Nope. The President’s pardon power under Article II, Section 2 of the Constitution is limited to “Offences against the United States.” Most state governors have the power to pardon state offenses, but many are limited (e.g., in Georgia, you have to complete your sentence AND pardon must be recommended by the pardon review board before the governor can issue a pardon).
Federalism. It is why both the state and feds can prosecute you for the same crime. If you are acquitted in state court you can still be prosecuted in federal court and visa versa and no double jeopardy defense will apply.
Hoffa got tried by a federal court.