Will the inquirer be held accountable for his actions with the married woman if her husband does not forgive him, and does that mean entering Hellfire despite his repentance?

You are obligated to repent to Allah and have a good opinion of Him. This type of sin usually does not permit seeking forgiveness from the wronged person, due to the potential for discord and strife. You must seek refuge in Allah so that He may appease your adversary. The statement that if the husband does not forgive, one will inevitably enter Hell, is false. On the Day of Judgment, the wronged person will receive their right through the retribution of good deeds and bad deeds, or Allah will appease them.