[OneWingedShark]

That's not correct; Obama would not be the first governmnet official found to have held an office for which he in fact was not eligible, and under the de facto officer doctrine actions taken by him would not be void.

De facto officer doctrine can only be cited by ignoring the Constitution's own requirements placed on the office; i.e. to invoke it is deprivation of rights under color of law. (The rights being deprived are those of every citizen to a government which follows its binding Constitution.)

[Meet the New Boss]

Yes but Obama and any other government official who deprives someone of a civil right by virtue of an official action is subject to the penalties under the statute 18 USC § 242 you cited and also the remedies under the analogous provisions of 42 USC § 1983 WHETHER OR NOT he is ineligible.

[old republic]

...under the de facto officer doctrine actions taken by him [Obama] would not be void.

That's not true. It's entirely unclear whether the De Facto Officer Doctrine--which by the way is not part of the constitution-- would even apply here. Not to mention, in Ryder v. United States (1995) the Rehnquist Court unanimously ruled that the De Facto Officer Doctrine did not apply to the judges who had been unconstitutionally appointed to the military court in question, because, amongst other things, those appointments had been made in violation of Article II's Appointments Clause. Relying on this SCOTUS precedent one could reasonably conclude that the De Facto Officer Doctrine does not apply when an explicit violation of the Constitution occurs.

To argue that the De Facto Officer Doctrine would apply even when it explicitly violates the Constitution is to essentially argue that the constitution is meaningless and unenforceable. Such a doctrine would also logically suggest that the constitution is not in fact the Supreme law of the land, since it can be disregarded with impunity by the government--which would also ironically be a violation of the Constitution's Supremacy Clause.

Applying the DFOD in the face of a direct constitutional violation would absolutely subvert the law in the name of maintaining order. It would also encourage the violation of the law since those impersonating an officer under the color of law would be able to get away with their acts being upheld even if they explicitly violate the constitution. This doctrine would ultimately subvert the rule of law in the name of saving it. It's a choice between sacrificing law and principle (the constitution) at the expense of order and convenience, or choosing temporary order and convenience at the expense of law and principle . Take your pick.