How did Ford pardon Nixon when the US Constitution says presidents can't pardon for impeachment?
"The President… Shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment."
There's only one exception to pardons: impeachment. There's no exception that says "and also if the impeachment was not yours".
So it's a fragrant violation of the US Constitution.