Answering some of your specific points, as best I understand them:
It sounds like, and please correct me if I'm wrong, the notion that a uniquely and exclusively American cultural and national identity (1) clearly and definably exists and has existed for the majority of our history,
I believe for the last century we've had a distinctive culture that, as I said above, both foreign and domestic observers could plainly see.
I think when we were a more homogenous people, roughly 89% white/European, 10% black, and 1% "other" (native American, Hispanic and Asian all included there), we were a more united and unified nation.
Read DEATH OF THE WEST, or even more so STATE OF EMERGENCY by Pat Buchanan for the statistics and history selectively omitted from the mainstream press, that is overwhelmingly pro-"multiculturalism" and pro-illegal immigration.
I frankly don't see why it is "good" to take a homogenous nation that is largely racially and culturally one ethnic group, and through massive influx of 1.1 million largely third-world immigrants a year (only 17% of which is white) Balkanize it and turn it into a multicultural/multiethnic tower of Babel, turn it into fragmented subcultures that each have their own news and entertainment, and barely tolerate each other. 50 years ago, we were the United States. Now we are Yugoslavia, Kosovo, Iraq. A divided mess of immigrant minorities who won't assimilate. Three days ago on Tucker Carlson's show, he interviewed a guy in California who is part of a movement to gather 550,000 signatures to have California secede from the United States and become a separate nation! Does that sound like a benefit of racial tolerance and multiculturalism to you?
I've said it before: there is no such thing as "multiculturalism", there is only the transition period, as one culture transforms into and is replaced by another. If we permit that to happen, then shame on us. You sound like you have contextualized it into an intellectual box where you are willing to permit that, or even say it is right, because we have not been "beneficial to all ethnic groups".
(2) is both inclusive of and beneficial toward all ethnic groups who reside here,
While obviously there have been historic wrongs to minorities, I believe the United States has done more to right these wrongs than any nation in history. And it frankly infuriates me to be dragged over and over the coals about racism from decades ago, or even over a century ago. That world does not exist anymore.
And I frankly think "equal opportunity" racial set-asides and easier standards for blacks, for example, only do more to create division and a condescending contempt for black America, that thinks it is entitled to jobs and universities, without having to do the same work.
Your (2) point reeks of the presumption of racial inequality, and ignores the incredible racial progress.
On the converse side, you have Asian students who, like whites, are denied opportunities to universities because there are "too many" as a ratio of the population who are high performers, and are thus passed over for lower performers who are black and Hispanic.
We can only come together as a culture when our society is truly color-blind, and only considers performance. My observation as a student was not that these "equal opportunity" underperformers either appreciated or tried harder to be worthy of what was handed to them.
My father (a white guy from rural Pennsylvania, formerly a steel worker) joined the Marine Corps during the Korean war and went to Penn State after on the G.I. Bill, and was the first in his family to graduate college. And worked his ass off despite his never having been a great student in high school, and graduated Phi Beta Kappa in electrical engineering.
While I was more of a B student, I worked while in college to pay my way, mostly as a busboy, waiter and bartender.
and (3) supersedes other concerns in guiding policy decisions.
Again, that has no specifics I can connect to anything, to even guess what you are asking.
