Is the repentance of one who apostatized and reached the point of disbelief accepted, when he fears his repentance may not be accepted due to the appearance of the Hour's signs and his despair, citing the verse "When they saw Our might" and Ibn Kathir's interpretation that repentance is not accepted upon witnessing death?

You have misunderstood Ibn Kathir's words. He clearly states that whoever's soul has not yet reached the throat, their repentance, whenever they repent to their Lord, is accepted. The one whose repentance is not accepted is he who has witnessed death. So your repentance is accepted as long as your soul is in your body. Beware of despair, turn to your Lord, and strive in His obedience. The one who repents from a sin is like one who has no sin, and the door of repentance is open and will not be closed in anyone's face until the sun rises from the west.