Yep; he would have to answer truthfully and in full; and then if they discovered after the fact that he lied to them about anything then that would be a NEW breaking of the law not covered by the scope of the pardon.
Yeah, no that would be lying under oath/perjury. It'd be picked up in a heartbeat and he'd be sent to prison for as many times as it took to help jog the old memory.
"I don't recall " can't be construed as lying, it was used in one of the most well known congressional hearings of Reagan and Ollie North during the Iran /Contra Hearings.