I'm not absolutely sure what you mean by "if Japan had defeated the United States." There was no possibility of that in August of 1945. I assume you mean if Japan had repelled the invasion by the United States, which was certainly a possibility
I would go further than you. If the USA had successfully invaded Japan, the American casualties would have been so high that the rebuilding of Japan would never have occurred; the American people would not have allowed it.
If Japan had defeated the U.S., the Japanese would have shown none of the forbearance, kindness, benevolence, and mercy toward the conquered Americans.
In fact--American forbearance, kindness, mercy, and benevolence shocked the conquered Japanese, who fully expected the cruelest possible brutality--which is exactly what they would have shown had they been victorious and which is why they were so shocked at American forbearance, kindness, benevolence, and mercy.
The Rape of Nanking would have been an afternoon tea party compared to what they would have done to the Americans,
And the Japanese knew it. That's why they were so pleasantly surprised at the Americans' treatment of them.