Will the people Trump pardoned, such as Joe Arpaio and Scooter Libby remain vindicated in the event of Trump's impeachment? Are there any circumstances under which their charges could be reinstated?
Will the people Trump pardoned, such as Joe Arpaio and Scooter Libby remain vindicated in the event of Trump's impeachment?
Any pardon that an impeached President makes prior to being convicted on an impeachment remains valid.
No President has ever been convicted following an impeachment, so there is no case presenting exactly this fact pattern. But, many other federal officials in the judicial and executive branches have been impeached and in no case did the impeachment invalidate any act taken by the official convicted on an impeachment prior to a conviction on an impeachment by the U.S. Senate.
(Strictly speaking an "impeachment" is a formal charge authorized by the House of Representatives which the U.S. Senate tries and may convicted upon, but "was impeached" is commonly meant to mean convicted on an impeachment and I am reading it in that sense. Certainly no act of a President who was impeached but not convicted on an impeachment charge, such as Bill Clinton, has ever been invalidated on the grounds that it was made after the President was impeached but prior to a conviction on an impeachment charge.)
Are there any circumstances under which their charges could be reinstated?
The federal charges which were pardoned cannot be reinstated by the federal government. See, e.g., U.S. v. Wilson, 32 U.S. 150 (1833).
The President does not have the authority to pardon state crimes or conduct that is criminal under state law. The U.S. Constitution, Article II, Section 2, Clause 1 states in the pertinent part: "The President . . . shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment." The U.S. Supreme Court upheld this reading in Ex parte Grossman, 267 U.S. 87, 113 (1925) ("We have given the history of the clause to show that the words "for offences against the United States" were inserted by a Committee on Style, presumably to make clear that the pardon of the President was to operate upon offenses against the United States, as distinguished from offenses against the States.").
So, if the conduct of someone who was pardoned by a President also violated state law and the statute of limitations has not run on state charges, new state law charges could be brought.
In Joe Arpaio's case, the pardon was for criminal contempt of a federal court order and there is no corresponding state charge that could be brought.
In Scooter Libby's case the pardon in 2007 was for perjury and obstruction of justice in the investigation into who leaked the identity of CIA officer Valerie Plame, and while there are state perjury crimes, a state is probably not permitted to prosecute a perjury case involving a perjury and obstruction of justice in a federal proceeding on federalism grounds.
It is also likely that any applicable state statute of limitations has run on that more than twelve year old conduct. For example, the statute of limitations on misdemeanor and felony charges would have run in Maryland, but in Virginia, the misdemeanor statute of limitations would have run, but there is no general statute of limitations for felony charges in Virginia. But, without a detailed review of the facts of the case it is not immediately obvious in which states the conduct giving rise to the offenses took place.
If the conduct took place in the District of Columbia and/or abroad, and hence was outside the jurisdiction of any U.S. state, no U.S. state would have jurisdiction to bring charges under any circumstances.
Incidentally, Libby's sentence had already been commuted even though he was not pardoned. This is the second time he has received executive clemency for the same offense. Also, as an aside, the validity of Libby's initial commutation of his sentence was upheld in the face of a court challenge to its validity. U.S. v. Libby, 495 F. Supp. 2d 49 (D.D.C. 2007).
A President can continue to exercise his power to pardon until he is removed from the office.
The House of Representatives impeaches. The Senate can then vote to remove from the office. Until the Senate votes to remove from the office (which requires 2/3rds of the Senate to convict), a President is a fully-fledged head of the executive branch and can continue to exercise all powers of his office.
If the Senate fails to convict (as was the case with Bill Clinton), the President remains in office and continues to exercise all the powers of the head of the executive branch.
None of the actions taken by a President as a head of the executive branch are reversed if a President is convicted by the Senate. This is not exclusive to pardons. It is also true of signed-but-not-ratified treaties (they remain signed), signed executive actions (they remain in effect unless reversed through new executive actions), etc.