How can it be just for the "innocent" husband to pay the price for his wife's infidelity after her repentance, and will the husband be rewarded for supporting a child who is not his, and what is the ruling on the questioner not marrying due to this hadith?

One must be careful not to describe the marriage contract as unfair, because that would be attributing injustice to divine legislation, and Allah Almighty is the Most Just of all those who are just, and He does not wrong people even an atom's weight. If a wife gives birth to a child on the marital bed, the child is attributed to the father to preserve the child's right. However, if the husband knows that the newborn is not his child, he is not forced to accept him, and it is legislated for him to perform li'an (oath of condemnation) with his wife and disavow the child's lineage, under certain conditions. The Sharia has protected the child's right by affirming his lineage, and has given the husband the right to disavow it through legitimate means. The questioner should not succumb to the whispers of Satan that turn him away from marriage and push him to accuse the Sharia of injustice. Rather, he should seek refuge in Allah, choose his wife well, and fear Allah concerning her.