I don't dispute many of those individual facts. I think it highly likely China has overcounted (or more likely, deliberately misstated) their population by ~100 million. I agree it's highly likely China's population has been in slight decline for several years now.

The 2020 Chinese census claims a population of just over 1.4 billion. Let's say that was overstated by 200 million, and it's really 1.2 billion.

Let's also assume that the Chinese birth rate remains at record lows or even drops further (2021 seems to be the record low so far).

I think you should also assume that Chinese life expectancy will continue to increase (as it has been, and is), and that the Chinese government will force people to continue working into older age.

To hit a population of 600 million by 2050, in just 28 years, the population would have to drop by 21.5 million every single year.

Barring the extremely unlikely events you mention--major famine and major disease--that's pretty close to impossible. Even at the height of the great leap forward, China never saw that level of deaths due to famine or other government-related reasons.

So, my tl;dr, declining population yes, demographic bomb, yes, major problems ahead, yes, but 50% population loss in less than 30 years? No way.