Would a person be considered an unbeliever, eternally abiding in Hell, if they committed an act that takes one out of the fold of Islam—while being unaware of it—and then trustworthy scholars issued a fatwa that they had not left Islam, but their fatwa was, in reality, mistaken?

One who is ignorant does not disbelieve due to what emanates from him out of ignorance. An example of this is the request of Moses' people to their prophet to make for them a god, and the request of some Companions from the Prophet, peace and blessings be upon him, to make for them a Dhat Anwat.