What is the ruling on congregational prayer in a mosque that deviates 45 degrees from the Qiblah, and is it obligatory to return to the correct Qiblah, especially since the Imam is aware of this and believes there is no obligation to return to it?

The prayer of someone whose qibla is off by less than 45 degrees is valid, because the obligation is to face the direction (of the Kaaba), not the exact عين الكعبة (Kaaba itself), and a slight deviation does no harm. This is what the jurists have stipulated; a significant deviation is one that strays from the direction, like facing east or west. The basis for this is the saying of the Prophet, peace and blessings be upon him: "What is between the East and the West is a Qibla." It is preferable to adjust the mosque's qibla and to bring the matter to the attention of the authorities.