What must I do with the final amount of usurious interest received upon breaking the deposit, what is the ruling on spending it, and what about the previous interest that was taken and spent, given that it constituted a primary source of income?

What you have taken from interest, believing it to be permissible, or following the fatwa (definition) of someone who permitted it for you, there is nothing due from you regarding it. As for what you have taken after knowing its impermissibility, it is not permissible for you to benefit from it; rather, you should spend it in charitable causes.