Is expiation required for someone who swore falsely upon the Mus'haf under duress, and then later confessed the sin?

The general rule is that swearing a false oath is a major sin. However, the Shari'ah has permitted it out of necessity to avert harm or achieve a legitimate benefit that cannot be attained otherwise, provided it does not cause harm to others or infringe upon their rights. Therefore, there is no blame upon you for this oath, as long as it was to prevent being killed or something similar.

You should have used tawriyah (ambiguous wording) in your oath. If you did not use tawriyah, then you must repent and seek forgiveness, and it is prudent to offer expiation for your oath. The expiation for an oath is feeding ten needy persons, or clothing them, or freeing a slave. If one cannot afford this, then one must fast for three days.