First off, I'd like to know what are the chances of democracy becoming a tyranny in the United States, so that gun ownership can be justified through this argument. Aren't the political institutions, checks and balances, democratic traditions, the rule of law not trusted enough so that you can buy an AK-47 on every corner without any restriction to take out the FBI, or buy a sniper in case you would feel the urge to shoot the President in the face?
It's a low chance. That doesn't mean we shouldn't prepare or even think about it.
Also, you cannot buy an AK-47 unless you have tens of thousands of dollars and the time to fill out a lot of paperwork.

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.
Armed citizenry resisting was the case a couple times in the 1800s. Right now it is unlikely. Times will change however, and the future may be different.

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.
Police or SWAT? Sure - they can be resisted easily. SWAT teams rely on surprise, they're not soldiers. As for Marines, perhaps you ought to take a look at how many there are (not that many) and how big the US is (huge) and how many people there are (a great deal). Marines can't be everywhere. Look at the trouble they had in Iraq, a country of far fewer people. The military cannot control the population of the US if even a fraction rebels.
Four, I'd like to know why do you think it's reasonable to uphold a law that allows extremists (right- and left-wing alike) to form paramilitary groups and train themselves to overthrow the federal government. Why is it such a good argument?
Because I want somebody to be able to overthrow a tyrannical government. Why should I care what they're doing so long as they don't actually attack someone?

Finally, why can't US citizens comprehend that a "well-regulated militia" should be read in a historical context of the Revolutionary War? Of course, it has its roots in English history, but still. Times change, so do laws.
It is read in that context- where every male was a member of the militia and well regulated meant well organized and armed. And no, laws do not change over time. They mean the same thing until they are rewritten or repealed.
All in all, common sense says it is a very weak argument indeed. Please come up with something else that makes sense.
Anti-gun folk always say they have the 'common sense' position because they don't really have any facts. If this country rebels, people aren't going to march out and find Marines to fight toe-to-toe with. Maybe you ought to stop thinking about how an idiot would stupidly fight.

The Repubs don't back gun control, and these days it looks as though the Dems don't either. Dead issue.
I'm afraid it's not. There's some dems and liberals for whom it is the main issue. They aren't getting anywhere right now, but that doesn't mean they won't try again. They must be kept under close watch, and the struggle for gun rights must continue.

CR