Is one who did not ensure the deceased’s eyes and mouth were closed, due to their ignorance of the matter, sinful? And is expiation required of them?

There is no sin upon you, nor expiation for what happened. Closing the eyes of the deceased and binding their jaws are supererogatory acts, not obligatory ones whose omission incurs sin. You exerted your utmost effort in fulfilling this supererogatory Sunnah, so you will be rewarded, Allah willing, and there is no blame upon you for what you were unable to do.