What is the ruling on praying in a mosque that was built before the grave within it, knowing that there is a shrine over the grave in the mosque, and knowing that some people face the grave in their prayer, while others are certain that there is no buried person in it?

It is not permissible to build mosques over graves, nor to introduce graves into mosques after they have been built. Prayer is not valid in mosques built over graves. The prohibition of this is out of fear of exalting other than Allah the Almighty, whether there is a buried person or not. The Prophet, peace and blessings be upon him, forbade praying at certain times due to resemblance to disbelievers, so how about praying in mosques built to exalt graves?