Is a deceased father sinful who acquired money mixed with unlawful [elements] from usury without knowing of its prohibition, and what should the heirs do with this mixed money, and is it lawful or unlawful, or is it obligatory to disburse the estimated unlawful portion of it through ijtihad?

Your father, may Allah have mercy on him, as long as he tried to avoid usury and fell into it out of ignorance without negligence, then it is not possible to rule him sinful or this money unlawful for him. This is because scholars explicitly state that usury which its owner takes out of ignorance, then repents immediately upon learning of its prohibition, this money is lawful for him, based on the Almighty's saying: "So whoever receives an admonition from his Lord and desists, he may have what has already passed." And if we say that your father owned this money, then it is lawful for you, and there is no need to dispose of it, though it would be better if you gave out the equivalent of the unlawful portion as a precaution.