This is a game where you climb up and then dismantle the ladder. Don't imagine the US will support other countries acquiring nuclear weapons! It's like the five permanent seats on the UN Security Council: no one wants to dilute their power, so no one sincerely wants to increase their membership.
You underestimate the impact of a trade embargo! China's accelerated nuclear weapons development coincided with the famine, and I think the connection is self-evident. Do you think countries like South Korea and Japan, with limited natural resources, have enough uranium to support their research? Not to mention, can their meager food self-sufficiency rates withstand a food embargo? Without oil, modern industry will be lost, and a lack of fertilizer will further reduce food production. A massive famine is foreseeable! Energy shortages will further hamper nuclear testing.
So you should ask them: are they ready for this?
The weaker US gets, the more incentive it has that China's hostile neighbours and its allies get their own nukes so that they can act as the anti-China trouble maker while it sits back.
Plenty of think tankers and other people in the west support Taiwan getting nukes these days. They actively cheer for it. Why do you think that is, its cause they are starting to realize fighting a Taiwan war is no longer viable for US. But that doesn't mean they want to give up on Taiwan, they think without US support the best option for Taiwan is to get nukes.
Preventing nuclear proliferation is a secondary goal compared to preventing China's rise or ensuring China is busy trying to fight its smaller neighbours and do not have the ability to project power across the globe.
US already implicitly supports Japan and SK getting nukes. Especially plenty of Trump-leaning "realists" think US should get out Asia Pacific alliances but allow Japan and SK to get nukes.
So, no I don't US or Europe will be against Japan, SK, Korea or Australia getting nukes. I expect them to support this openly in the future. It will be up to China to prevent it. and they have to use Military force to do it.