Is the testimony of two individuals from the group that they departed from Arafat after sunset on the 9th of Dhu al-Hijjah sufficient to waive the penalty (dam) for someone who doubts whether they departed before sunset?

The person mentioned is not obligated to do anything. The testimony of two men regarding their departure after sunset is sufficient, and doubt does not harm him, especially since the authorities prevent pilgrims from leaving before sunset. As for your going to Arafat on the evening of the eighth, this is contrary to the Sunnah (definition). The Sunnah is to spend the night in Mina on the night of the ninth and then go to Arafat on the ninth day. However, nothing is consequent upon this [your action]; because spending the night in Mina on the night of the ninth is a Sunnah, not an obligation.