Does anyone know if this is a British thing, or just an error? It's from the copypasta'd source, not steve.

You are now aware that Noah Webster decided British spelling was bullshit and set about to make American less phonetically tortuous:

Noah Webster was struck by the inconsistencies of English spelling and the obstacles it presented to learners (young and old alike) and resented that American classrooms were filled only with British textbooks. The spelling reform featured in his first dictionary, A Compendious Dictionary of the English Language, was based on the author's combined vision of logic and aesthetics. He changed the –ce in words like defence, offence, and pretence to –se; abandoned the second, silent "l" in verbs such as travel and cancel when forming the past tense; dropped the "u" from words such as humour and colour; and dropped the "k" from words such as publick and musick. The "publick" readily accepted many of these changes and just as readily rejected some of the others.

As to the meat of your observations:

Sorry, why does it matter how Chauvin murdered Floyd?

Because extenuating circumstances impact what crimes Chauvin was charged with and what he could be convicted of. Meanwhile, the American legal system is predicated on every defendant being able to avail themselves of a vigorous defense. Which leads to the following observation:

There was no defending what Chauvin did. We've all seen the footage.

Right - which means a "vigorous defense" is reliant on some pretty left-field jiggery pokery in order to instill a "reasonable doubt." I mean, mental exercise: you have to defend Derek Chauvin in a court of law. He is not pleading. He's either going to be an example for the Left for the next 10 years or an example for the Right for the next 20. You swore an oath. Your professional and ethical obligation is to do your level fuckin' best to get Derek Chauvin acquitted. What's your play?

If that's not Kobayashi Maru enough for you, let's presume you're a thinking, feeling human being with an awareness of the stakes, of the emotions, of the symbolism and of the importance of this trial. That vigorous defense you're presenting - can you do it in such a way that you're not going to inflame things further? You defend Derek Chauvin in just the wrong direction and he's suddenly Stacey Koon.

I've seen a number of people point out what a horrific thing it is that such an obvious verdict was ever in doubt. As it is, things didn't get worse today, which is about the best we could hope for, I think.