Is what happened before Islam, in terms of dating and engagement, considered a legitimate marriage, and is it permissible to pray for a righteous husband, but that the aforementioned person not be a husband in Paradise?

Congratulations to the questioner and her family on their embracing Islam. It should be clarified that marriage is not concluded merely by a verbal offer and acceptance, but rather by what is considered valid in the religion and customs of the couple, as ruled by Ibn Taymiyyah. Furthermore, even if the marriage were valid, the conversion to Islam of one spouse while the other remains in disbelief dissolves the marriage upon the expiry of the iddah (waiting period), as mentioned by Ibn Qudamah. Therefore, there is no need to worry; she should resume her life. There is no harm in supplicating for a righteous husband, and a woman in Paradise will be with her husband from this world if he also enters Paradise with her.