Is the prayer of one who prayed to the left of the Imam out of ignorance valid? Must it be repeated? And does its ruling differ between the scholars and the Hanbalis?

If there is no worshiper to the Imam's right, your prayer is invalid according to the Hanbalis, but valid according to the majority of scholars. You may follow the opinion that it is valid. Ibn Qudamah said in Al-Mughni: "If the congregant stands to the Imam's left, and there is someone to the Imam's right, his prayer is valid. But if there is no one to his right, the prayer of the one who stood to his left is invalid according to the Hanbalis." The majority of scholars believe that a single congregant should stand to the Imam's right, and that if he stands to his left, he has contravened the Sunnah.