A divinely appointed Imam is a human...
A divinely appointed Imam is a human, but being human does not mean that he could not have any authority over other human by the permission of Allah. The closer the human is to Allah, the more authority he will have. Closeness to Allah is obtained through obedience and piety. When one reaches that level of perfection, he does not desire anything except Allah's orders. Imam is not divine, but he is fully supported by divine power. He has been given authority as the Quranic verses below testifies.
This authority has come from Allah and is also controlled by Allah. Another example would be to consider a company with a president and some managers and employees. The one who is closer to the president (in his position or any other aspects) has more authority than the others. This authority is given and is controlled by the president at any time. This authority is not in parallel with the authority of president, and remains as long as the president wishes to.
The authorized person does not feel independence and can not do anything against the order of the president, otherwise the position will be taken away from him. The president has appointed that person to carry out a task. Similarly, a divinely appointed Imam is NOT considered a parallel power beside Allah, and his authority is NOT independent of Allah since Allah does NOT relinquish His Majesty and Sovereignty to ANYONE.
If He gives His righteous Servant a certain power or authority, He will still have control over that individual.
Quran testifies that Allah has ASSIGNED some Imams with authority to guide mankind: "And We assigned them Imams who guide by our authority and We have inspired in them the doing of good deeds." (Quran 21:73) Also: "And We appointed from among them some Imams who guide by our authority since they were patient and believed firmly in Our Signs." (Quran 32:24) Moreover, on the commentary of Quranic verse: "And lo!
verily I am Forgiving toward him who repents and believes and does good deeds, and afterward he accept guidance [20:82]", Ibn Hajar mentioned that it is narrated from Imam Muhammad al-Baqir (AS) as well as Thabit al-Lubnani that, the end of verse means "he is guided to the Wilayah of the Ahlul- Bayt." (al-Sawa'iq al-Muhriqah, by Ibn Hajar al-Haythami, Ch. 11, section 1, p235). Allah, Exalted, also said: "O' you who believe!