He wouldn’t have been able to best the mirror, AND dumbledore was on his way back to the castle pretty much immediately.

He couldn’t best the mirror puzzle with Harry Potter being present (the only way he had any chance of cracking the mirror puzzle) because dumbledore showed up without any aid from the rest of the team. If he had to do so without Potter in the same amount of time before the principle returned… he would’ve failed.

Except they only put the stone at risk because they went out.

If Potter had stayed home, nobody would’ve found the stone. The stone was perfectly safe until Potter showed up.