The "spoken only in hell" thing? I think that was just a bit of bravado. What's the point to the genocide? We aren't talking about Nazi Germany. Most Americans are racist against the Japanese and want revenge, fine. But they don't see the Japanese people are a threat in the same way the Nazis did the Jews. The US will accept the total surrender they demand. If they want revenge, they can make the peace as harsh as they want. No "reverse course", no supporting Japanese re-industrialization, even minimal civilian food aid I could see. But I can't see them going through the expense of bombing and shelling the islands, of putting US soldiers and sailor's lives at risk, just for the glee of killing a few more "japs". The government of the US did not base its decisions on revenge fantasies.
Oh, and it doesn't matter if the Emperor has control over the military or not. I'm talking about seeing him as useful post-war. By then the IJA will be long gone. What matters is if they think the Emperor can be useful in the occupation, not in any sort of peace process.
I agree completely about the genocide fantasies. The US will still accept a Japanese surrender. In fact, given what the Germans may be up to, the Japanese may still end up being slightly less bad than the Nazis on the US evil-o-meter. What we don't know in this TL is what the Emperor can actually say or do that would make him seem that useful. I think it will need to be a bombshell (pun intended)