All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
First of all, may Allah reward you for being a reason for that girl to embrace Islam. We ask Allah to put this good deed of yours on the scale of your good deeds on the Day of Judgment. Whoever calls to guidance will have the reward similar to all those who were guided through him.
Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet said, "Whoever calls to guidance will have a reward similar to all those who follow him without their reward diminishing in any way…" [Muslim]
You do not have to marry that girl. Fulfilling the promise is not an obligation according to the view of the majority of the scholars; it is only recommended.
If you are able to be just between her and your first wife, then marry her as a way of fulfilling the promise, and because you both love each other.
Ibn ‘Abbaas, may Allah be pleased with him, narrated that the Prophet said, "The best solution for those who love each other is to marry." [Ibn Maajah]
If you fear that marrying a second wife will lead to a greater harm, then, in this case, it is better to suffice yourself with your first wife only. For more benefit, please refer to fatwa 257272.
You can help this girl by looking for a pious man to marry her. You must cut all kinds of relationship with her, because she is a non-Mahram (marriageable) to you, and this will close the door of temptation between the two of you, especially given that you love each other.
Finally, what you mentioned about the miscarriage is not necessarily a punishment for a sin; rather, it may be just a test from Allah.
Allah knows best.