Is it permissible to pray to be united with a married woman in this world and the hereafter, knowing the impermissibility of the previous relationship?

What you did by engaging in a relationship with that woman is not permissible for you, especially what involved corrupting the woman against her husband. Your supplication for the woman, her husband, and her family for forgiveness and good things are commendable acts. As for your supplication that Allah unites you with her in the Hereafter, the detailed explanation for this has been provided in Fatwa no. 81204.