Is his failure in marriage and work a result of his sins during his study period, and does the saying of Allah (SWT) "So as for the orphan, do not oppress [him]" (Quran 93:9) apply to the orphan girl whom he was with but did not marry?

What you have mentioned in terms of actions indicates that studying in non-Muslim lands poses a great danger to those whose faith is weak. This is in line with Allah’s saying: "But there came after them successors who neglected prayer and pursued desires; so they are going to meet evil" and His saying: "So woe to those who pray, [but] who are heedless of their prayer" and His saying: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way" and His saying: "O you who have believed, indeed, intoxicants, gambling, [sacrificing on] stone altars [to other than Allah], and divining arrows are but defilement from the work of Satan, so avoid it that you may be successful."

As for your question about the girl, the description of an orphan ceases upon reaching puberty. If you forced her into immorality while she was not yet an adult, that falls under Allah's saying: "So as for the orphan, do not oppress [him]." In general, Allah expiates sins through sincere repentance, as He Almighty said: "O you who have believed, repent to Allah with sincere repentance" and He said: "Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."" Therefore, hasten to repent, feel remorse, and resolve never to return to such acts.