If the founders intended us to be able to overthrow the government by force any time we felt like it, why did they put in a clause to allow Congress to call forth the militia to quell just such an insurrection?
Because an insurrection could be mounted by those wishing to overthrow a free country. Stopping that and preserving the free country would be a legitimate use of the militia under the 2nd amendment.
Conversely, some politicians could gain control of the country and try to turn it into a police state, and being unable to change things with traditional manner (say the government doesn't allow or follow elections, or recognize the right of the people to petition their government), then the 'radical' model of the 2nd amendment is called for.
Take what you're saying a few steps further:
If there was no 'right' of the citizens to overthrow a corrupt and tyrannical government, what limit on their abuse of power would there be? By that viewpoint, there would be none: A government could continue to seize power and deny rights until it's a completely police state with no representative government.
Do you think the founders didn't understand that? I'm pretty sure they did, which is why they spoke so strongly about resisting tyrants.
Remember, according to them, the ultimate power rests with the people. So a corrupt congress can call up the militia, but the people who make it up can still say "no" with their guns.
It is right and proper for people to overthrow an illegitimate government. It is right and proper for a legitimate government to protect itself from being overthrown. In case of conflict, the Constitution is meant (among other things) as a guide for people to determine which side is the right one.
Such an occurrence would only be an "insurrection" if the government were able to successfully "quell" it. If not, if there were enough of "we the people" supporting said insurrection, then history would simply record it as another successful war for independence and freedom from an oppressive government. The Second Amendment is simply the mechanism to deny a minority (government) the power to oppress the majority (we the people) by having a monopoly on the use of force.
Put more simply, we DO have the right to try to overthrow our government by force any time we feel like it, and the government IS authorized to enlist our fellow citizens (the militia) to stop us. The historical verdict as to whether our cause is right and just will be determined by who wins. If the government is truly and sufficiently oppressive, then we (who are also militia) will be able to persuade enough of our fellow citizens (militia as well) to join us to prevail. If our grievances are unreasonable and unpopular, however, the majority of the "militia" will remain loyal to the government, and we will be defeated.
This is simply the concept of checks and balances carried through and applied to the use of force. There is no contradiction, as you seemed to think in the first paragraph above.
"That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness."
Our Declaration of Independence says we have the right abolish and reorganize our Constitution whenever Government gets out of control. The Constitution is the best attempt of some pretty smart people to keep the government from getting out of control. The bill of rights includes the tools we need to control government, including arms to "alter or abolish it."
If you don't recall, our current Constitution is the second form of government. The founders threw out the first one in 1789.