2) Nuclear weapons are the only physical weapon that can check the US Government. That's why they shit their paints when a basically inconsequential country such as North Korea launches a potential nuclear rocket.
3) In the 1700s guns were the best weapon of the time. Nuclear weapons did not exist. Civilians were able to own basically military grade weapons and could theoretically rebel and potentially win.
4) In current days, if you wanted to legitimately make the claim of government check on power with civilian weapon ownership, you'd at least have to argue for military grade, fully automatic weaponry ownership rights by civilians. Not to mention the ability to stockpile explosive weapons and missiles. A bunch of civilians with gimped semi-auto ARs and hunting rifles would have been the 18th century equivalent of the right to bear bow and arrows or pitchforks and shovels.
2) Yes, that's why the USG had no trouble brushing aside the Iraqis and people in Afghanistan who didn't want us there.
3) Irrelevant. Nuclear weapons are so hopelessly overscaled you can't actually use them - they're only good for threatening. It's not like Washington is going to nuke Los Angeles.
The reality is every successful indigenous revolution requires a schism in the military. That's the way it's always been. But an armed citizenry means a whole lot fewer military units have to defect.
4) Soldiers in the US army don't get fully automatic weapons at the individual level - those are squad level weapons. The fancy drones and missiles require a fully functional economy to create and maintain. Once a civil war starts all those supply lines dry up.