Secondly, I'd like to know what are the chances of a foreign power invading the United States, and an armed populace resisting the invaders, so that gun ownership can be justified through this argument. If this would happen, wouldn't it be a massive failure of the defense forces? And if the US defense forces failed to defend the country, the invaders would presumably possess weapons the general populace couldn't counter with a bunch of machine guns.
The invasion of a foreign power into the United States need not simply be a total invasion of all areas of the United States. Were Cuba to invade South Florida, that would be a "foreign power invading the United States" and could conceivably be either harassed or countered by a combination of partisan gun-owners, detached armed forces, and police forces until National Guard and Army troops could throw the Cubans back into the sea.

An armed population will have zero chance against a foreign military invasion, see Iraq/Afghanistan. Their only chance is, like in Iraq and Afghanistan, to win a war of attrition.
So an armed population can win against a foreign invasion. The statement is entirely predicated on the assumption that our foes share the same moral standards we do. If they have censored news and no problem killing Americans, then we have a serious problem that will result in the total defeat of America no matter what we have.

Thirdly, I'd like to have your bets on the chances of an armed militia resisting the police, the SWAT and the Marines in the case of an armed revolution against a tyrannical government. I'd bet all my money on the Marines.
If an "armed militia" consists of a couple disaffected homeowners who own hunting rifles, then I would bet on the Marines, Police, and SWAT. Were it a tyrannical government which is generally unpopular then the "homeowners" might eventually gain support to overthrow the government. Especially if we were to enlist the aid of countries who don't like the "current tyrannical government".

Meh, strawman. They will obviously NOT be assigned to posts where they have the chance to shoot their own families. And families could join their side too.
Impossible. You are seriously suggesting that the Marine Corp is going to go through it's lists, and then move troops around so much to prevent Marine units from being posted in "home-areas" is not only highly impractical but it would also ruin unit cohesion and prevent the Marines from operating effectively in a counter-insurgency. While Marines are some of the best soldiers in the United States, we aren't talking about heroes. If that was the case, we wouldn't have friend-on-friend or civilian casualties would we?

As for proof of my argument. Look at any civil wars. Not everyone joins the side of the state. Armies and generals can join the opposite side as well. Look at all the cases of revolutions around the world, same happens there as well. They don't just obey their master, especially if the master is very unpopular, even with them.
Agreed.

Zombie Terror Outbreak happens, how will Joe Bloggs defend his farmhouse Left4Dead style?
Greatest support for gun-rights.