I'm still in Ukraine, and at Russia's current rate of advance they will reach Kyiv some time in the Winter of 2094. That's not actually a joke either, it's a calculation of their rate of advance measured against the miles left to go before they reach Kyiv. In the past 18 months, their territorial gains have quite literally not been enough to build a cemetery large enough for the men they've lost while taking it.
So, no. I don't see it falling to Russia. They've launched eight failed offensives against Kharkiv alone already.

As for China having 100 nukes pointed at South Korea, I doubt that. Frankly, China barely has 100 nukes at all, and using 100 on South Korea would leave them with nothing left to use against the US, though they would have invited a nuclear retaliatory strike. And further, North Korea's interceptors can shoot down 100 missiles with no problem. If South Korea isn't sure of that then they've got the manufacturing ability to produce more.
Or buy Iron Dome batteries from Israel.
Or study Ukraine's "intercept by drone" technology, which is working amazingly well (Russian Shaheds are 0 / 1435 in the past 7 days on hitting their target).
South Korea's not a third world country, they're a techno-power. They've got US protection but they're also capable of building their own defenses. China will fail humiliatingly if they try it.