Excuse my ignorance- I'm not wise in American History and I don't understand the question.
Surely American Isolationism ended with PH?
Or is this double blind?
Isolationism is more of a policy that has come in and out of favor throughout American history than a specific time period, America hasn't been officially isolationist since PH but its public has been and there have been more isolationist candidates and Presidents since then. He's asking if OD would make the U.S more or less isolationist.
To answer OP I think that it would make the U.S public far more isolationist. Assuming a worst realistic operation downfall, we're talking potentially 1 million U.S casualties here. That's double the civil war. That's way more than the rest of WWII, several times more. The fighting would be brutal, it would be like a combination of Iraq, Vietnam, and the European theater. I think not only would the U S casualties have a major effect, I think the psycological damage on the American psyche of seeing the effect on Japan would also be significant. Remember that 50% of Okinawa was died during the invasion of Okinawa. We could easily be talking at least 30% of the Japanese population here. We'd have the bombings for longer, maybe nukes, plus the fighting and inevitable famine. This fighting would be far more brutal than Europe or eventually Vietnam. While the war itself is obviously fairly popular it could really traumatize the American public.
Also, think of the resources that would have to go into rebuilding this Japan. The country would be a wasteland filled with an angry and impoverished people. They would definitely have to remove the Emperor permanently, possibly execute him. The entire Japanese political class would have to be recreated. This would take a collosal investment to actually rebuild Japan.
I think it would force a more active U.S foreign oolipo but it would also make the U.S public more tired and mitigate the growth of the 1950s significantly as the U.S has significant casualties and lots of investment that has to go into rebuilding Japan.