09 would be to late.
Not really; the election can be ruled illegal anytime. If it was illegal, it was illegal, regardless of when it’s determined that it was illegal. Even if it’s after January 20, it’s not too late. He’d simply not be the POTUS because the election was declared illegal. It’s against the law to run unqualified candidates; it’s against the law to swear in unqualified candidates; it’s against the law for electors to vote for unqualified candidates — the law in all these cases being the Constitution itself.