If Obama was shown to be ineligible to be POTUS it would not necessarily cause a civil war, but certainly a constitutional crisis. Every executive order would be invalid and all bills passed by Congress and signed or vetoed by the impostor would likely be challenged as illegal. There could also be criminal charges against Obama and those who helped in this conspiracy. The Democratic party would be thoroughly discredited as a willing partner to this fraud.
Well, they might be challenged. But more than likely not overturned. SCOTUS picks, etc. would stay. He would just go away ... but only after impeachment and conviction. No court in the land has the power to remove a sitting President.
Under the 25d Amendment, Congress could declare him unable,incompetent, or unfit and sort of set him aside in favor of the Vice President. Never been done.
And of course, he could resign. So could have Joe Stalin.