What is the ruling on closing the mosque approximately 30 minutes after Fajr prayer, and does that deprive someone who wished to earn the reward of a complete Hajj by remembering Allah until sunrise?

The solution is to speak with the mosque attendant to permit sitting in it. If that is not possible, then one should go home and perform dhikr (remembrance of Allah) until sunrise, and then pray two rak'ahs.

Al-Qari mentioned in "Al-Mirqat" that this reward is attained by one who moves from their prayer spot and continues in dhikr until the sun rises, then prays two rak'ahs.

The Sunnah indicates that if a Muslim is prevented by an impediment from a righteous deed they habitually performed, its reward will be written for them. In the Hadith: "If a servant falls ill or travels, what they used to do when healthy and residing will be written for them."

This applies to one who was performing an act of obedience but was prevented from it, and whose intention, were it not for the impediment, was to continue it.

The one who wishes for good, and is keen on it, if it was their habit to perform it, but something prevented them, their full reward will be written for them.