Is it obligatory to pay Zakat on the deceased father's usurious wealth from interest earnings before distributing the inheritance, or is it considered a debt upon the heirs to be paid from lawful earnings?

There is no doubt about the prohibition of Riba (usury/interest). However, it may be difficult to ascertain whether the father paid his Zakat from usurious interest, due to the commingling of Riba with the principal in the account, and because currency units are indistinguishable. If he paid Zakat from his wealth which includes both Riba and its principal, it cannot be said that he paid his Zakat solely from Riba. The fundamental principle concerning a Muslim is that he does not pay his Zakat from unlawful wealth. Therefore, the heirs are not required to pay Zakat on the inherited wealth; rather, they are required to dispense the amount of the usurious interest.