Just caught up, love the timeline. Plausible and gripping.
My only quibble is with the big turn here, which is the Soviet decision to nuke China. The author explained that move well and I certainly do not see it as ASB. But it rang a little false to me, at least within the context of this story. The Soviet regime was panicking and was facing strategic collapse, so I agree they'd have wanted to do something.
But why risk alienating China, with whom USSR still has a non-aggression pact, even if they're doubting Chinese intentions? A first strike on China would still seem to the Soviets like a good bet for provoking Reagan into a first strike on themselves, which after all was what drove the Soviet decision to go to war in the first place.
What seems more likely to me is that they'd have given Reagan the ultimatum they eventually did give him - but before an attack on China, not after. Basically, end this war on terms that are acceptable to the USSR, or else face nuclear attack. That puts the onus for armageddon on the USA and potentially wedges Western Europe apart from Washington - at least in Moscow's mind.
Reagan would be facing a pickle of a decision. My guess is he goes on TV to say he won't negotiate with nuclear terrorists, and then proceeds to do precisely that, laying out his own terms for an armistice. Eventually one is reached, after lots of threats and brinksmanship and a nuclear exchange is avoided. The endgame looks like a mix of 1962, where the USSR eventually blinks, and 1918-19, where the USSR and Warsaw Pact collapse right before, during, and after the cessation of hostilities. NATO promises Moscow that they might get a decent deal out of the peace settlement, while counting on the fact that Moscow will not be in charge of much at all within a matter of weeks, allowing the West to more or less impose its terms.
Still, that's less dramatic than an actual nuclear war, and you did a great job showing how it would have played out on that dark night of February 17, 1984.