Is it permissible to rely on the ijtihad of a scholar when there is evidence that contradicts it, and if it is proven that the first Adhan during the caliphate of Uthman was at Zawwal, then is our giving of the Adhan before Zawwal considered an innovation (bid'ah)?

The matter of the first Adhan for Friday prayer is broad, and its purpose is to prepare people for the prayer. There is no definitive proof that Uthman (may Allah be pleased with him) ordered it to be given after Dhuhr. Some scholars have permitted this Adhan before Dhuhr, and Sheikh Ibn Uthaymeen (may Allah have mercy on him) and a group of scholars inclined towards the preference of this opinion. This is a very reasonable opinion. One must be cautious of objecting or rushing to declare something wrong or innovative, as scholars have considerations that may be hidden from those who have not fully comprehended the issue.