Re: China successfully tested its version of the W-88 warhead between 1992 and 1995
Nukes do not cause much destruction by their explosion. The direct explosive destruction radius of the 5000 US nukes would not suffice to destroy China. The destruction via nukes is by their radioactive poisoning. It's a massive flow of neutron radiation that is released and that creates radioactives isotopes over a very large region. There's no chance to filter all these isotopes that kill off humans. One idea was to wait out most of these isotopes in special shelters. Well, bad news is, while most life not in shelters gets wiped out, the nuke and the radiation leave enough radioactive heavy metal lying around to make consumption of produce of the land deadly for centuries to come (the reports about Polonium poisoning give you an idea how that feels like). The Japanese were lucky as "their" warheads were not that refined neutron emitters.
Furthermore, nuclear warheads have the typical mushroom cloud of an explosion that transports material from the ground into higher atmosphere. For a nuclear warhead this can be very high in the stratosphere, giving all inhabitants on a earth a share of the poisonous nuclear downfall. Since radioactive testing started we had to totally reset the C-13 dating globally (!) despite the tests not being intended in kind and scale to wipe out life on earth. Nukes are MAD and the only option of actual use is tactical size for very high value targets. I would agree that US missile defence opens a window of opportunity for this option against China, while at the same time development of thermobaric weapons and cluster munitions close the gap between conventional weapons and tactical nukes. The Chinese second strike doctrine is a reply to the total annihilation threat of the MAD days of the Cold War. Flexible nuclear use, as envisioned by the US can pose difficult questions whether to risk an all out strike with MAD over one small tactical nuke. Missile defence adds a layer of uncertainty to the effects of that decision. Chinese discussions of some level of first strike tactical nukes might help to seal this opening in Pandora's box.
It's usus that if you don't have nukes, you have chemical and biological weapons up your sleeve to pose at least some deterrence. Unit 731 had assembled enough know-how with their gruesome methods to commit mass murder of some scale even in the US and be it via suicide agents after the "end" of the war. The problem with all these methods is that they turn things messy, they don't solve it and armed violence is for solving a dispute. It helps to read old man Mao a bit, he was a brilliant military leader.
As to peak oil population decimation. If such a catastrophy was to strike home, the US controls the blue water undisputed and has slight problem securing the major oil sources. They can make sure that they have enough and others pay dear or die because they own the SLoC distribution network by right of arms and there's no other network with the slightest chance to compete (look at modern global transport statistics). It's very important to develop methods to safe energy, produce storeable green energy and tap new energy sources such as deep sea methane in order to secure Indian and Chinese survival and prosperity in the future.
Japan and Germany are considered capable to produce nuclear weapons and Germany and Italy were partners almost to the finish in the French nuclear program (with shared information). They are allowed under the non-proliferation treaty to strike back nuclear if under such an attack. Because the requirement of nuclear weapons is not a big blast, but a maximum of poison distribution, they are far easier to manufacture than the early scientists thought who worked on the explosives aspect and had only limited fissile and fusion material at their disposal.
You might research WWII a bit more, all nations involved had developed chemical and biological weapons of mass murder. It seems only the Japanese and the US used this kind in war with the US taking a loophole by introducing something new and the Japanese continuing concepts of the Conquistadors of America. Even someone portrayed as a madman as Hitler(in my opinion he was not mad, but quite a cunning and unscrupulous politician, who reflected what influential interest groups wanted Germany to do) denied his generals to use the chemical stockpile.