Yes, Trump did have the power to pardon, preemptively pardon, and grant amnesty. Sources: Article II, Section 2, Clause 1 of the United States Constitution; Ex Parte Garland, 71 US 331; US v Klein, 80 US 128; Carlisle v. United States, 83 U.S. 147
the Court has indicated that the power may be exercised at any time after [an offense’s] commission,8 reflecting that the President may not preemptively immunize future criminal conduct.