OP's story is unfortunate, but this is tame compared to some of the things I've seen or heard of in the past five years. If this is TC-worthy, I've seen enough dirt to fertilize the Sahara.

This is a case of company and person who didn't work out, but it doesn't leave me feeling that Microsoft was unethical or mean-spirited about it (which is more than I can say of many, many companies). Badly run, sure, but not evil. Microsoft offered him a severance, and they gave him ample warning. Besides, if the company (or, at least, the part of it into which he landed) was that bad, why'd he stick around for so many years?

Why look for work elsewhere when I could coast from meeting to meeting, uttering and typing meaningless busywork. I could not relinquish that kind of comfort.

I may be unusual, but I get nervous and antsy in this kind of "comfort". It just makes me feel like no one is getting anything done and disaster is coming. Also, experience has led me to conclude that when the group is underperforming, the "rebel" rather than the cause of the underperformance is the one to get smited, utterly regardless of individual performance. So, environments where nothing is getting done scare me. Even if I am individually doing great, there's going to be blame to be allocated, and the fact that I'm individually an asset to the team is no shield.

In my mind, working itself is fun. Even when difficult and frustrating, actual work is not stressful, except in very rare moments of crisis. Those interminable sitting-down "stand up" (do people not know what the fucking words mean?) meetings that many companies have, on the other hand... fucking intolerable. I would not be "comfortable" if my calendar had 5 hours per day of status meetings and pointless chatter on it. I'd go insane.

My planned and promised promotion was cancelled.

That should have launched a thousand resumes. Or at least five or six. Why delay in getting ready? As OP has learned, they won't. Corporate "loyalty" is dead. Just don't "job hop" if you chance upon the one company in 20 that actually treats its employees decently and knows what it is doing... because, unless you know where you're going and trust the people you'll be working with, you probably won't find another. Job hopping is only stupid/dodgy when you leave a good company for a pay raise.

Official HR warnings were sent.

Anyone who is not looking for another job after the first "official" warning is sent is a fucking moron. It doesn't even have to come from HR. Negative verbal feedback from the boss might be genuine constructive criticism. He might be trying to groom you into a leader. Negative written feedback, such as an email? That is not ever to your advantage. If your boss is genuinely trying to improve you or groom you for something better, all such feedback will be verbal. Hostile email? Then get the fuck out. The case is being built.

For the record, PIPs (which is what the "dubious case" sounds like) are a kangaroo court and they can be emotionally draining. The trick is to recognize what they are and not get emotionally drained. (The reason to avoid emotional drain is not to save your current job; that's over. It's to do well in your transition and next job.) Don't get emotional. It's just a damn game, and the only way to win is to get out. Forget the shitty sweet talk. This manager doesn't want to "improve" you. He wants to get rid of you. Your job is to leave on your own terms before you get fired-- or, if that can't be done for some reason, engage in the legalistic fighting but realize that the best-case scenario is for the PIP to be ruled "inconclusive" and, unless you can transfer after that, have the manager PIP you again. Almost no one ever passes a PIP. Most people leave; the rest either get fired or the PIP is ruled "inconclusive".

I was offered 12-weeks’ pay for an amicable departure.

Take it. More than enough time to get another job.

Instead I decided to escalate the thoughts above to the highest echelons of Microsoft.

Terrible move. Like, Ned Stark in Game of Thrones. Managers and executives are fundamentally tribal. Most of them will protect their own, at any ethical or business cost except their own skin. If you go against one manager in front of another, your credibility is shark-shit at the bottom of the Mariana Trench.

Likewise, never threaten to leave a job over managerial misbehavior. You'll just get fired. You don't threaten to get another job (unless you've found one and are trying for severance). Just get another job.