Did Imam Ahmad ibn Hanbal permit swearing by the Prophet, and did he interpret the verses of the divine attributes, and is that considered to be from the creed of the people of Sunnah, and is the one who does it deemed an infidel if it was a mistake?

The imams have agreed that swearing by a created being is forbidden and not binding, based on the Prophet's (peace be upon him) saying: "Whoever takes an oath, let him swear by Allah or remain silent." Some have differed regarding swearing by the Prophet (peace be upon him), but the preponderant opinion and that of the majority of imams is that an oath sworn by a created being is not binding.

As for allegorical interpretation (ta'wil) of Allah's attributes, such as the Face and the Hand, this is a fabrication against Allah. It is obligatory to affirm them for Allah as they have been reported, without distorting their meaning or likening them to the attributes of created beings. This is the belief of the Ahl al-Sunnah (definition) wal-Jama'ah.