Looks like proving that Hunter used his father's name corruptly for business deals has been done, though the standard of proof in criminal court, very appropriately, is very high.
Proving that Joe received $$ in substantial amounts (e.g. more than 10s of thousands of $$ in "repayments" for funding Hunter's drugs and hookers habits), that he knew where this $$ came from, under what terms, and Joe knowingly/willingly participated in Hunter's deals, beyond a reasonable doubt, is several high mountains not yet climbed, AFAIK.
Obviously, all these painstaking processes - investigative reporting and criminal investigations - should have begun a year and a half ago, but there was a Bogey-Trump to defeat.