Why is a person held accountable for their actions if guidance and misguidance are in the hand of Allah, and if people's capacities differ while duties are the same, and if hearts are turned by the will of Allah?

A Muslim must believe that Allah Almighty does as He wills and rules without an overseer. He is just, wrongs no one, guides whom He wills, and misguides whom He wills. However, Allah holds a servant accountable for his actions because He granted him intellect and will to distinguish good from evil, and religious obligations became binding upon him based on this choice. Therefore, your statement "Why are we held accountable for what we have no control over?" is incorrect; a person chooses his actions, and Allah's foreknowledge does not imply predetermination.

As for the variation in abilities and souls and the requirement of unified obligations, Sharia does not equate people in their religious duties; those with excuses have rulings that suit them, such as tayammum (dry ablution) and shortening prayers. As for differences in zeal and will, these are individual matters and people are required to develop them, and Allah increases the guided in guidance.

Regarding the fluctuation of hearts, a person is required to work and choose, and the Prophet, may Allah's prayers and peace be upon him, taught his companions that everyone is facilitated for what he was created for. The questioner must have a good opinion of Allah, for Allah does not waste the reward of those who do good nor does He wrong His servants. An evil end is for those who are negligent or hypocritical.

Therefore, one must occupy oneself with what is beneficial and strive to attain obedience, following the example of the Companions, whose knowledge of divine decree did not prevent them from working and striving.