What is the ruling on a young man eating from money which he knows for certain or preponderantly believes came from usurious loans, or from commercial transactions with a semblance of usury, or from work whose primary income is unlawful?

There is no blame upon you for benefiting from the money borrowed with usury from your uncle, because the prohibition of a usurious loan is related to the borrower's responsibility (dhimmah), not to the money itself. Your uncle should be advised to repent from usury. Moreover, it is not forbidden to deal with or benefit from the money of someone whose wealth is mixed between lawful and unlawful, rather it is only disliked.