At the time he gave the red card he was sure. It was after giving it that the doubt crept in and talking to the assistant confirmed it wasn't just a convincing appeal by the defending team, that it was a good tackle.
It can. After the ref has made a decision, he has until play restarts to change his mind, if he feels it necessary.
Because he thought it was a penalty. The ref is in charge, if he thinks he sees an offence he has to call it, he can't stop and go check with his assistant every time he is...