Is it permissible to benefit from the mother's money if its source is unknown and it is suspected that she acquired it from usurious transactions, knowing that she is an employee, has debts, participates in associations, and does not disclose the source of her money?

There is no harm in benefiting from the money your mother gives you if her work is permissible, and as long as it is not known that the money itself is unlawful. There is no obligation to investigate its source. The unknown is like the non-existent, and the default assumption regarding what a Muslim possesses is that it is his property. However, if a person is known to have unlawful wealth, then dealing with him should be avoided out of piety. As for a discreet Muslim, there is no doubt in dealing with him.