(1) Sure, I'm not arguing that the Chinese economy is less vulnerable to a SOH closure than the US. I do think the US government is much more vulnerable to economic pressure than the Chinese government is (especially in an election year that even before the war was shaping up poorly for the ruling party), and any calculus the government makes needs to include this. If this was the goal of the war, I think we would also see significantly different targeting and messaging than we do now. If there was a ceasefire tomorrow, it's unclear that China would be the outsize loser here.
(2) Again, sure, but Iran can clearly sustain it longer. They've read their Clausewitz and properly understand this as a contest of political will, which they have much deeper reserves of than capital or munitions. Anyone with any power in the Iranian regime knows they have no offramp.
Absolutely agree that Trump cares strongly for his legacy, maybe more than anything except for his self-image, but the most important part of that legacy is being recognized as both popular and a winner--I would argue that these are far more important to him personally than US power and influence on the world stage (shutting down USAID, for example, was a massive blow to US soft power, and the NATO infighting that he initiated is still probably a net negative for US hard power, even if it has had a positive impact on European defense spending and self-sufficiency). He also clearly wants to see that legacy established in his lifetime (hence the obsession with having things named after him). It's hard to imagine this being a particularly effective way to increase long-term US power and influence relative to China, particularly in a way that will generate positive sentiment within the US--especially among the majority voters who favored his populist-isolationist political platform.