I thought they had to be convicted for a Presidential pardon.
Nixon wasn’t.
No, the president can pardon anyone at any stage of the proceedings. USUALLY, they don’t pardon until after sentencing.
That hasn’t really been determined by the courts. Gerald Ford gave a preemptive pardon to Nixon, but nobody challenged it so it’s still a gray area. The pardon power is very broad, so there is a good chance that a preemptive pardon, at least for specific acts, might hold up.
Nixon was never tried. Pardon is a plenary power.