It would actually have been a smart move, I think, for Japan to join in the war against Russia early in 1941...and to leave the USA alone. This would probably have led to a Russian defeat, and Japan and Germany would have carved up Russia, as you say.

However, it wasn't going to happen. The Japanese had had their fingers burned in a limited war with the Russians in the late '30s (the Nomonhan Incident). Although they bested the Russians in the air combat over Nomonhan, they did quite poorly against the better-armed Russians on the ground. The Russians had far superior tanks and heavy artillery.

This made the Japanese extremely hesitant to get in a large land war with Russia. They were eager to avoid that possibility at all costs. Thus Hitler got no help from them at all, and they moved south instead to fight a naval war with Great Britain, the Dutch East Indies, and the USA.

Hitler was a very popular man in Germany in the late 30s, as you say. He had an extremely loyal populace (for the most part), because he had greatly improved the economy and strength of Germany in the years before the war. They people who supported him weren't technically crazy...they were just regular people who had confidence in his leadership, that's all. At the time...it seemed to be working...and everyone loves a winner, don't they?

German career officers did not expect Hitler to declare war on the USA after Pearl Harbour, and they must have been shocked and quite worried about it when he did. They already had more than enough on their hands fighting the UK and Russia.

As for Japan, Stalin's belated and opportunistic attack on them in Manchuria in the last few days of WWII may have had just as big an effect on their decision to surrender as the 2 atom bombs did. The one thing they were most afraid of (other than harm to their Emperor) was a major land war in Asia with the Russians.