The one child policy only really mattered in the cities, rural China had different rules. There is also no incentive for China to lie, quite the opposite, underreporting their population would be a boon for their success on the global stage: imagine if they are achieving what they achieve, with half as many people?
Yes, except that China also uses its population as a military threat. It going down would take away some of the impact of that. So it always needs to go up, to reinforce it.
Does it? Russia has 1/10th the purported population of China, lost most of their military aged men in a conflict that has exposed Russia's supposed military might as a work of fiction and yet the west remains scared senseless of Russia because of the nuclear threat. China has nuclear weapons, whether they have 10 million or 100 million men they can send to the frontlines to absorb bullets is irrelevant to their national security.
Europe is right to panic. It exposed that Europe was relying only on the US and the general world order. And the US turned out to be unreliable.
Imagine this scenario: tomorrow the Ukrainian front collapses, and Russia rapidly captures a significant part of Ukraine. Then the Ukrainian government gets Venezuellaed by Putin (maybe with Trump's help), and the new government becomes loyal to Moscow.
Then a new charismatic military leader deposes Putin and forms an alliance with the Ukrainian army against Europe. With rhetoric like: "Look, Europe just used you. They never gave you enough weapons to win against Russia but just enough for a stalemate. They were giving Putin hundreds of billions for gas and oil, too afraid of cold weather while you were dying on the front lines. They hoped to kill both of our countries. Now let's join together and show them how the war should be fought properly". And then a battle-hardened 500000-strong army marches towards Kaliningrad, locking Poland and the Baltics behind the front lines.
It's an exceedingly unlikely scenario. But not impossible. And it's not the _only_ similar scenario anymore. There's also Turkey with a dictator dreaming about writing his name in history books. Serbia is getting more anti-EU.
Mostly in the past before they were well industrialized. When you had India with over a billion people as a threat, it was a good measure. Now most of the surrounding countries have fallen below population replacement rate excess population can cause issues with economic growth in places resources and space are constrained.
Many companies setup branches and sent IP to China in exchange for access to those billion consumers. Fewer consumers means a company might target India, etc. instead of China first.