I think a relatively accurate model of the people's opinion towards intervention might be quite simple: it is good if we win relatively swiftly and bad if we lose and/or don't gain anything, and the opinion at the time is shaped (and over time altered) based on their estimate of the outcome, but no politician says it that way so it is always cast as black and white pro-war/anti-war.
In the current case, I think many Americans, even Democrats, recognize the regime in Iran as a threat that needs to be dealt with somehow (a deal or an intervention). Their worry is the cost and ramifications, not some ulterior principle. If Trump brings home a win and some oil to boot soon-ish, you're going to see positive sentiments more clearly. If this drags on, the backlash will be there, and will be phrased as "MAGA never wanted the war" and along your lines of isolationist promises not kept.
The most important thing to understand about Trump and conservatism in general, by far, is that there is one central principle that underpines the entire ideology: hierarchy. Going back to the time of kings and nobility and clergy, through to the present day.
"Conservatism consists of exactly one proposition, to wit: There must be in-groups whom the law protects but does not bind, alongside out-groups whom the law binds but does not protect."
One set of laws for the people higher in the hierarchy, and one set of laws for the people lower in the hierarchy. Things that are okay for them to do are not okay for you to do. Wars started by Democrats are bad. Wars started by Republicans are good. They know this is not convincing rhetoric to anyone who is not part of the in-group, so they lie about their reasons and play games with words. This, however, is what they truly believe.
It is why every action they take appears hypocritical to their opponents, but in actuality, it is perfectly consistent with their values - it is good when they do it, because everything is good when they do it, and it is bad when somebody else does it, because everything is bad when somebody else does it. It is why "the only moral abortion is my abortion". It is why the exact same policies executed by different presidents will have the same approval rating by democrats, but a completely inverse approval rating by republicans (eg 40% of Democrats approve of either Obama or Trump striking Syria, while 20% of Republicans approve if Obama does it and 80% approve if Trump does it). It is the single consistent trend through all of their policies. They know exactly what they were voting for, and that is for the man who represents their hierarchy. The games he plays with words are part of the platform.
Edit: I have rewrote the message quite a bit, apologies if anything doesn't make sense.
It may be the case that his base is still just following him and supportive of whatever he does.
But the number of people who voted for him vastly exceeds his “base”, and the entire MAGA movement is basically predicated on a form of isolationism, or at least not pro-intervention. Part of the reason it became popular was as a reaction against the wars in Iraq and Afghanistan.
So I don’t think it’s as simple and one dimensional as you paint here. Which is exactly why I think it’s a systemic problem: many people probably voted for him because of the campaign promises of being against foreign wars.
My impression is that most of his voters are selfish and couldn't care less for other people's woes (migrants, sexual abuse victims, Iranians or whatever), but will care if his antics hit their own pockets. I'm not American so I may well be wrong, though.
Their support is not the result of a rational calculation of self-interest, and never was. If it was, a base of rural and poor people would never be supporting a coastal city New York elite born with a silver spoon in his mouth as "one of them". But they do, because he is one of them in the way that matters to them. They are fighting for something larger than themselves, and are completely committed to a cultural war for social hierarchy.
> if gas prices and general inflation spike hard, as is nearly a given if Trump doesn't back out from the war?
As an aside, I don't think there is any backing out of this war. If somebody launched a missile at your country and killed hundreds of schoolgirls, and destroyed ships on diplomatic missions while leaving the survivors to drown, while also assassinating your country's leader (but not out of any intention of liberation), would you just let things go because they stopped bombing? Of course you wouldn't. Your country would continue to retaliate. And it is trivial to punish America. Even if America unilaterally decided to "declare peace" and withdraw from attacking Iran, Iran has every reason to continue locking down the gulf and making Americans pay the price. Unlike with tariffs, there is no backing down from these price increases even if Trump gets cold feet. But, even so, there is no reason to believe it will move the needle on his base. There is already talk of "short term pain for long term gain" among those who realise this.
Expecting to hold any promises just because they were said and got him where he wanted is a bit naive, don't you think? Or does the idea of 'but now he will act completely differently to his entire prior life!' makes any sense to you?