Does my manipulating a girl's feelings and promising to marry her without genuine intention count as an injustice against me, for which there will be consequences on the Day of Judgment? And is my apologizing to her, without informing her of my true intention at the time, sufficient to clear my conscience?

The conversations and meetings that took place between you and the girl are among the forbidden pretexts. You did well by leaving them, and you are not unjust to her by cutting off the relationship. As for your lying to her and misleading her with marriage, it is forbidden and you must repent from it. You did well by seeking her forgiveness, and since she has forgiven you, ward off these whispers from yourself. She brought it upon herself by complying with you, but everything is by the decree of Allah. If the girl is religious and you are able to marry her, then do so, for marriage is one of the remedies for love.