Regarding 1), Dumbledore says he sent instruments back in time to examine the fight. Presumably he verified the troll was not Quirrel. (Even though the instruments may have been tricked, he might not believe it likely.)
Regarding 1), Dumbledore says he sent instruments back in time to examine the fight. Presumably he verified the troll was not Quirrel. (Even though the instruments may have been tricked, he might not believe it likely.)