How about, IF...he were found to NOT be qualified to be President (not being a naturally born citizen of the U.S.), and he were impeached because of that...would everything he ever signed into law be nulified and void?
Not exactly. If he is not Constitutionally elligible to be President, then he cannot be impeached. To be impeached, one must first be President. If he's not a Natural Born Citizen, then he's not Constitutionally President. And so none of the bills he's signed nor the Executive Orders he's issued would have any Constitutional validity. All those he's appointed to positions in the government would not be (nor ever have been) officers of the government—for example, Sotomayor and Kagan would not be Justices of the Supreme Court, nor would any of the votes they cast count.