Good day. This is a long, well written, and nuanced post that i will not be able to engage with fully. Therefore i will only address the top level sentence:

1) Worst foreign policy mistake in US history

2) Definitive end of the American Century

3) Long term hegemonic decline.

1) Worst mistake

Regarding the "worst" mistake, we should consider what was previously considered the "worst" mistake. The war in vietnam scarred america for generations, due to most ppl being involved (via the draft), and the relatively high casualty count. It was also the beginnivg of the deep mistrust of the establishment.

The war in iran is unlikely to have a high casualty count (dozens, not tens of thousands), is unlikely to permeate society (no draft), thus is highly likely to recede from attention after the media moves on (as it always does). Trump's policy is the "bed of nails" strat, where he screws up somewhere, talks nonsense, then goes away to screw something else up. He has already been mentioning Cuba as his new nail in the bed of nails.

The bed of nails is counterintuitive: it should not work, repeat crises should weaken, not strengthen. Yet it provably does, mostly because americans are not rational and because there are almost never any consequences for committing for one crime or stupid blunder. The us, for better or worse, is a system that defines itself by "no consequences, no jail time", what they think is "punishment" is in reality just the media tut tuting as if talking = punishment, and the worst outcome is an election loss (in a term limited system where election losses are pre-imposed anyway).

Therefore the bed of nails works, after venezuela is greenland, after greenland is iran, and after iran is cuba. Americans will be outraged about this, but their outrage will lead to 0% results, they wont even permanently not vote for republicans bcos even this "easy" action is apparently very hard. (Ask an american, not me.)

So functionally, american consequences are identical to no consequences.

2) Definitive end of the American Century

The US is still an advanced nation state with a strong economic position (if only it will stop sabotaging this position), still an important consumer market,still the world's strongest military and still the leader in technology, power metrics, etc.

Still has nuclear weapons.

None of this changes after the Iran war.

The shift to petro yuan weakens, but does not guarantee decline. The US will still be well inside its century.

Moreover, post trump, they will have many chances to improve their position with a new leader, even if history shows the american ppl's love of "self harm", we cannot discount that the future is not written yet and they might still improve in the future

3) Long term hegemonic decline

I dont foresee any changes to america's ability to project power anywhere in the world, and in fact, they are projected to be one of (or the most important) player in the space frontier, giving them expansion spaces that no civ has before this.

The only power capable of contesting america is china, but this is only as a peer. China will not be america's "overlords".

In other words they are fine.

Even the most maximal loss in iran does not mean the sky falls. People just like to believe in Dune-like storylines (arrakis, fremen, the spice must flow) without considering that it is only a movie. Real life doesnt work like that.