In reality everybody knows that Ukraine was disarmed for a non-proliferation reasons no matter what was promised them.
Africa is in flames right now because they have supplied everyone who had sticks and stones with rifles, and guys who could only afford rifles got their tanks, APCs and aircraft dirt cheap.
Ukraine have sold a carrier to the Chinese for some freaking $20 million, given them carrier aircraft to learn technology and operation, and where do you think rogue regimes learnt a rocket science?
Now imagine if Ukraine had the nukes. Even Mexican cartels might could have couple by now.
Ukraine could have been left with, say, 20 nukes, and an inspection program by an outside multi-country team, leaving no chance the weapons would go anywhere (except to a target if needed.) But, no, someone wanted Ukraine essentially defenseless, and that goes beyond nukes. This continued on into the recent past: Read up a little on Viktor Yanukovych...
Getting rid of the rest of Ukraine’s nukes WAS a good idea, but not backing up the assurances that went with that deal will almost certainly help lead to a significant nuclear war - hard to say where - in the next 50 years or less.
As for the rest:
A) You VASTLY underestimate the capability of countries or groups who are determined to be armed, to be armed, and heavily so. Try studying some history. Almost any time period will do.
B) Remove Ukraine from the equation, and the world’s “arms situation” would be essentially unchanged. It’s just like drugs - where there are buyers, there will be sellers, as well as those brewing up their own. Only in the nuclear arena can there be any real curtailment of proliferation, and even that is heading into the toilet.
C) The Chinese are perfectly capable of building their own carriers and carrier aircraft, and arguably would be better off (ie., more dangerous) on their own, even if it took a bit longer.