Is it obligatory to resume friendship with someone who has wronged me grievously, and will I be held accountable for not befriending him again, knowing that I have forgiven and pardoned him, but do not wish to befriend him?

You are not obliged to restore the friendship to its previous state; it is sufficient for you to return and offer greetings. This is because forgiveness without offering greetings is not enough to end the estrangement and escape the warning mentioned in the Hadith.