Whatever you do, don't question any past election once it is certified, they advise, again, and again, and again. Certified elections are sacred.
I don’t think you get how this works. There is a set date of certification, and once certified, no further changes can be made. It works like this because the Founders of the US recognized that in the case of irregularities, it could easily take months or years to resolve things, and the issue was even worse in their day, when long-distance communications and travel could often take weeks.
This means that they were willing to accept that, on rare occasions, someone might be elected only due to screw-ups or fraud, but that doesn’t change the validity of the election. However, it’s not quite as “sacred” as you’re making it out to be; if there’s any reasonable possibility of the vote totals being significantly off (regardless of whether it would have changed the outcome) there’s still an obligation to continue investigating past the deadline, to remove the source of fraud or incompetence (via firing or arrest) if anything is found.
And if the candidate themselves were proven to be actually involved rather than merely beneficiaries, then there are nearly always remedies for that, depending on the particular office. And that remedy is never “the second-place winner automatically gets the position”, though depending on how it’s remedied that might be a *possible* result. (Key word being “automatically”.)
Though statutes of limitation are a thing, eventually rendering some lines of investigation moot. Even when/where they don’t apply, at some point there isn’t much reason to bother anymore: either there’s no longer anything to find, assuming there ever was, or anything you do find will merely be a historical footnote.