Will Allah hold us accountable on the Day of Judgment as a single soul, based on His saying: "And He created you and will resurrect you only as a single soul," or is there another proof for that?

The verse mentioned refers to creation and resurrection, not to reckoning. Allah's power encompasses the reckoning of all creation simultaneously.

As for the reckoning of people at one time, there is a detailed explanation: there is a difference of opinion regarding the reckoning of prophets and martyrs, but for others, their variations become apparent. The nation of the Messenger, peace and blessings be upon him, will be the first to be reckoned, starting with the rights of Allah, then the rights of His servants.