Is the prayer valid if it becomes apparent that the direction of the Qibla was wrong after relying on the guard's answer? And am I sinful for not verifying the direction myself, despite the presence of a nearby mosque?

You must repent to Allah Almighty for your shortcomings in attending the congregation, because congregational prayer is obligatory upon every able-bodied man. A slight deviation from the Qibla does not affect the validity of the prayer. However, a major deviation (such that the Kaaba is to one's right, left, or behind one's back) invalidates it.

If the deviation you experienced does not reach the point of invalidating the prayer, then you do not need to repeat it. However, if it reaches the point of invalidating the prayer, then you must repeat it. If a person with sight prays in a settled location facing a direction other than the Qibla, and then his error becomes apparent, he must repeat the prayer, even if he relied on someone who later turned out to be mistaken. This is because a settled location is not a place for ijtihad (independent reasoning) regarding the Qibla, due to the presence of mihrabs (prayer niches) and individuals who can provide certain information.