We're well within the realm of speculation but: The Iranian leadership has prepared for decapitation attacks which is partly why they weren't as effective. Their so called "Mosaic Defense" was built for that. They have seen what Israel dealt to Hezbollah. So they must have known this would be an option. There was also no question that at some point force was going to get used, at least by Israel. Israel accounted for about half of the firepower (and really 100% of the firepower on Tehran pretty much) and for the entirety of the decapitations strikes. Israel said on multiple occasions it will attack Iran if Iran didn't stop pursuing its nuclear program. With Hezbollah weakened that threat/lever was less effective. Israel was very worried about Hezbollah's retaliation. Israel had already decimimated Iranian anti-aircraft defenses. So all of this is expected, rational, and what Iran accounted for. I agree the degree of US participation was a surprise but not a zero probability event, certainly after their participation in the last round, and their massive military buildup.

Likewise the closing of the strait was no surprise. These sort of scenarios are planned for and there is zero doubt the closing of the strait was a scenario considered by the US and Israel military planners.

Not a ton we can say other than that. Maybe the US and Israel thought the blow would be so hard the regime would crumble. Maybe they thought Iran wouldn't dare. Maybe they thought that if Iran closed the strait they'd be able to reopen it by force. Indeed this could be where over-confidence, or incompetence, or inexperience, comes in on the US side. It's also that one can never fully predict how things would develop. There could have been over-optimism and under-estimation of the Iranians ability to withstand the air campaign or to effectively close the strait.

All that said, both sides are rationally pursuing their interests. Iran's regime wants to survive and it wants to keep building missiles and nuclear weapons and expand it's religious and political influence. The US and Israel want to put a stop to this before Iran has an arsenal of nuclear weapons mounted on long range ballistic missiles. Both sides will do their best to not tell you what they think or what their plans are (and the Iranians are definitely much better at this than the current US admin).