There is NO QUESTION the US would provide a full defense of Japan against any aggressive party.
The US has multiple military bases in Japan, with 35,000+ military personnel. Japan pays the US billions every year to support the US military presence there. Japan is also a too-big-to-fail economy (4th in the world) and US trading partner. And strategically, what do you think the US "pivot to Asia" means, if not defending close US allies in the Asia-Pacific from unprovoked aggression?
For over 60 years the United States-Japan Alliance has served as the cornerstone of peace, stability, and freedom in the Indo-Pacific region. The U.S. commitment to Japan’s defense under the U.S.-Japan Security Treaty of 1960 is unwavering. https://www.state.gov/u-s-security-cooperation-with-japan/Ukraine surrendered the sharpest tool in its arsenal for those assurances, its inherited nuclear arsenal, the world’s third-largest at the time. But the loss was broader than warheads; it was the surrender of a strategic future.
America first means America first. All politicians will say one thing and do another, always check the incentives…
>Seek immediate Security Council action to provide assistance to the signatory if they "should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used".Even if they managed to land they would probably be pushed off pretty quickly. Japan's military is more powerful than that of Ukraine, and the Russians are already having trouble supporting troops just across the border. There's no way they would be able to support an invasion force over water. I'm skeptical the Russians could pull that off without opposition, something they would certainly have in spades.
A land invasion of mainland Japan is so unrealistic that even the US in WW2 didn't attempt it.
Knowing those two facts, we can conclude that Russia will not be invading Japan as long as it is protected by the United States. The calculus is very simple when nuclear weapons are involved.
Also, keep in mind that D-Day was the largest amphibious assault in history and all they had to do was cross the English Channel. Russia invading Hokkaido would be suicide, the US nuked Japan rather than try invading the homeland to end WW2 and we controlled every island surrounding Japan at the time.