I believe that China requested a delay in the spirit of "dude, not during olympics plz"

That interpretation is inconsistent with reducing gas supply to Europe over a period of months to ensure gas supplies will be depleted after the winter of 2021. The logical conclusion from Russia’s military buildup (which intensified in October 2021) must have been that Putin plans to invade, ideally in late 2021 but at the latest in early 2022.

Why "must"? Why couldn't it have been a bluff, in which Putin wants others to think he will invade (unless given sufficient concessions) but if no concessions are given, he plans to end the "exercises" and pull back? I don't see how you could rule out this possibility using just the gas supply evidence. If he was bluffing, he would have wanted to make it as realistic as possible, which would have included reducing gas supply.

I can't remember the exact timing, but the UK and US governments/intelligence services were giving briefings to the press to the effect "Russia is preparing to invade Ukraine. They must back down." The earliest I have found is from November 2021, but I think it was covered before then. Not sure when they first said anything, presumably they knew before they said and they suspected before they knew.

I think that their was still a chance of buying peace with concessions until the day the tanks rolled. Russia asked for Nato to promise to never admit Ukraine. I believe they also made some other demands. Its not either/or when it comes to sabre rattling or genuine preparation to invade. An actually prepared invasion seems like a good threat to take into a negotiation. And if they still don't give you what you want you have this lovely invasion all ready to go. (Although less ready than they thought.)

Note: I have zero expertise in the situation, and this is just my attempt to make sense of the situation. 

Oct. 13, 2021: Putin claims that German approval of Nord Stream 2 was an "obvious" way to relieve natural gas prices. 

• Opposition to Nord Stream 2 by the USA has long been framed in a security argument. As an example, an Aug. 30 column (by Mark Temnycky, but he was far from the only person to see the strategic implications of Nord Stream 2) stated "The deleterious effects are easy to foresee: If relations were to sour, Russia could turn off the flow of gas to Europe, potentially leaving millions of Europeans without it... What’s more, Europe’s dependence on Russian gas could embolden the country to meddle further in Ukraine’s affairs, without fear of repercussions. That could lead to an escalation in the Donbas conflict — a yearslong Russian military incursion in eastern Ukraine that has led to the deaths of over 14,000 people and the displacement of nearly two million — or even a Russian effort to unite eastern Ukraine with Crimea."

Sept. 1, 2021: Biden and Zelensky meet for the first time in the Oval office to state opposition to "Russian aggression." The frame was as a way to reinforce ongoing US commitment to Ukrainian security even as the US pulled out of Afghanistan.

July 30, 2021: USA drops effort to block Nord Stream 2 due to the project being too advanced to stop.

Apologies for all the paywalled NY Times links, they just make it easy to search for articles within a defined date range.

Maybe the big takeaway here is this zero-sum game theoretic model:

1. World leaders are smart gamblers.
2. Smart gamblers bluff both to win a losing hand, and to trick others into playing when they have a winning hand. In other words, they cultivate strategic ambiguity.
3. Smart gamblers don't play games with negative expected value.
4. The anti-inductive nature of gambling means that even the best players will only have a small edge.
5. Since people don't play games they expect to lose, the game's players all expect to win.
6. If you're not playing the game, be ready to be surprised by the outcome.

Here's how I'd apply it retrospectively:

• "Policy" is the word for credibly and honestly telling somebody exactly what you'll do in a given situation. "Threat" is the word we use when we're truly not sure if the other player's holding a strong hand, or whether they're bluffing. "Bullshit" is the word we use when they other player is making non-credible bluffs. Nobody was bullshitting in this situation. They were making threats in order to extract policy concessions.
• It can be politically useful to accuse someone of bluffing for a variety of reasons. Bluffing is a smaller infraction than making a serious threat, so an accusation of bluffing gives you and the person you're accusing a way to back down. Claiming a threat is merely a bluff makes the threat/bluff more expensive to maintain, because they have to bolster it to make it more credible. It also buys time before you have to start taking the threat seriously. But you can only claim a threat is a bluff so many times before saying "you're bluffing" itself becomes non-credible. So when a world leader or major newspaper accused Putin of "sabre-rattling," we can't necessarily assume that's what they really think. It's a move in the game, not a straightforward assessment of the situation.
• It's rare but not impossible for a world leader to be a bullshitting idiot. It's more likely they're a shark, someone for whom their best strategy is to play down their competence and get their rivals to underestimate them. Donald Trump would be a great example. Putin, especially by 2021, was clearly not an idiot, and so we should have assumed that Putin never bullshits - he only makes threats.
• Nord Stream 2 and military buildups were a genuine threat. The energy threat was made credible by the fact of European energy demand and artificially constrained supply (i.e. by the anti-nuclear lobby and decarbonization efforts). The military threat was partly made credible by the invasion of Crimea, but on a deeper level, Putin wouldn't have threatened an invasion if there wasn't a serious possibility he might do so.
• Another genuine threat was military support to Ukraine, along with threats of sanctions. The US's powerful military and economic engine, its lower level of reliance on Russian energy, its historical willingness to get its military involved around the world, and its vocal support for Ukraine even during the withdrawal from Afghanistan made that threat credible.

I find it hard to think this way. But I think world leaders are really good at it.

I didn't believe an invasion was about to happen up until it had. At most I thought Putin would place Russian troops in the breakaway regions. I would have given less than 5% odds to a direct attack on Kyiv. Even with the benefit of hindsight I still cannot make sense of what transpired. Even if Putin managed to conquer most major cities, Ukraine would have become a major pain for him. I assume this is ideology trumping reason.