But see, no it isn't. The single most important message of that story is not that Diana Rockwell did anything for the amazons, it's that our Diana honors her. The amazons, particularly Diana, are routinely accused of arrogance. Of xenophobia. Of thinking they're better than us. Of looking down on us. Of all kinds of nonsense that completely misses the point. Diana Rockwell is the single most powerful refutation of that the mythos has ever had - not only do they not look down on us, they honor us so highly that their own princess wears the colors of one of us. Diana looking up to one of her gods, or to other amazons, doesn't begin to come close to that. Taking out the "Diana Rockwell is why I wear this" completely guts that whole story, removes its entire purpose.
That is not an origin, Bluefall. It's a condition. There is a huge difference.
... how's that any different than "last survivor of an alien race"? That doesn't tell you anything about Clark whatsoever. It doesn't tell you why he chose to be a hero, it doesn't tell you what his powers are, it doesn't tell you why he's a reporter or, frankly, why you should care about him at all. It's just who he is. Clark is the last of his race, Diana is the best of her race. I'm really not seeing any distinction there.
If we had had a long flashback story about how Lois Lane's grandpa was an astronaut who met Jor El through a wormhole and left behind a scarf with the letter "S" on it, and that inspired the Kryptonians to...blah blah blah blah, well, you can perhaps see how ungainly that would have been.
Well, really, we've had a dozen or more of those, and yes, they are ungainly. So? In the end, it easily pares down to "it's the crest of the House of El," which takes precisely one sentence to explain. Likewise, Diana Rockwell Trevor is a bit ungainly to lay out in full detail. However, in the end, it easily pares down to "she wears those colors to honor an American who died for an amazon," which takes precisely one sentence to explain.
If Diana's story is true, then the Amazons inspired the flag.
It doesn't matter who inspired who. Prior to your retcon, the American flag inspired the amazons. As of your retcon, the amazons inspired the American flag. Either way, Diana's wearing an American flag. Your point with the swastika actually makes mine. Both the Buddhist symbol and the Nazi symbols are swastikas. Which one came first is irrelevant. Either way it doesn't make either of them any less a swastika, or give either group any more or less claim to it. And even though the Buddhist symbol did come first, how many people in your audience are going to see it and think "oh, Buddhism" and not "oh, Nazis"? However few, it's still more than are going to look at Diana's costume and think "oh, star fields" and not "oh, American flag."
if you'll notice, most writers avoid it entirely
If you'll notice, most writers avoid everything that came before them entirely, regardless of how gainly it might be. However, Jimenez, WML and Rucka all managed to reference Diana Rockwell just fine without any awkwardness.
I don't read SuperFriends. This is something we've had in mind for a long time. But to be fair, it could just be a bit of cultural mythology.
Psh. Next you'll be telling me there's nothing sinister about aglets.