"You might make an argument either way that fighting to impose representative government is noble or not (or effective or not, etc.), but fighting to impose an oppressive regime is always evil."

So the Americans are evil to impose Karzai's regime on Afghanistan and maintain it by winking at massive election fraud? ;). No elections were held in South Vietnam because it was widely known in the American Administration that Ho and co would have won hands down.

"I doubt very highly if your notional American militiaman had sworn any oath to the King of England."

It didn't matter. For a subject (and pre independence, the people who lived in America were the subjects of George 3) to take up arms against his sovereign was treason, by the laws in place at the time, and the notions of honor in place at the time. Kings had "God given" rights to rule as they pleased.

Of course,in practice, rebellions happned all the time, noble or not, legal or not and if you won, you weren't a traitor anymore. viz the Shakespearean quote about how treason never prospers because if you prosper in treason then you can redefine the meaning to make yourself a hero. The Americans won. So they are "freedom fighters". If they had lost they would be mutinous traitors.

"in WWII, it was sometimes necessary to destroy entire cities in order to achieve legitimate military goals. "

And it was sometimes necessary to destroy Vietnamese villages to save them. Or waterboard and torture people to get information. Or drop an atom bomb on civilians to avoid casualties to your own armies in "honorable" combat against uniform wearing (but stubborn) foes. Use napalm against unarmed villagers. etc etc. Rendition and torture innocents. Protect mercenaries from prosecution etc.

If the people who do the defining of what a "legitimate military goal" is , can break the "laws of war" at will (the Geneva Conventions, which you quote also ban torture for example. By a strict interpretation, everyone from GW Bush down is a war criminal), and quote "military necessity" then they have no meaning any more except as political fig leaves.

All your examples of "dishonourable" fighting serves to illustrate my point that tactics which are effective against the dominant powers of the day are 'dishonorable' and as soon as they are adopted by them they become "honorable."

However if it makes you uncomfortable not to think of the US Army as some kind of paragons of honor and virtue fighting the forces of evil, go right ahead. To each his illusions.