Unless Obama were impeached and convicted, Biden would be VP. If Obama never qualified after becoming President-elect then Biden becomes Acting President but not actually President. I don't know if a presumptive President or a President elect can be impeached, but I don't think so. If 'President elect' is considered a 'civil Office' by virtue of being explicitly mentioned in the COTUS then I suppose so, but that is really, really stretching the definition. A gain, as Impeachment is limited to 'The President, Vice President and all civil Officers of the United States' I don't think the President elect can be impeached.
From that way I read the 20th, if BHO were disqualified, Biden would become acting president until someone is qualified.
And the Democratic controlled Congress would choose the President.]
Even if they left it up to the States, the Democrats control more state houses and Gov. seats than the GOP.
So, if you knock Obama out, you will end up with a President Biden, Pelosi, Reid or Clinton.