What is the ruling on praying in a mosque that was purchased with an interest-based loan, initially being unaware of that transaction, and how can one rid himself of usury when it has become so widespread?

There is no objection to praying in a mosque built with a usurious loan. The sin falls on the one dealing in usury. If those in charge of the mosque did not intend to deal in usury, then there is no sin upon them. Usury is forbidden in all its forms, and it is not permissible to borrow with it, even for building a mosque. However, whoever borrowed with usury is permitted to benefit from what was purchased with it, and they must repent.