No, I really don't think so. The principles are the same between all varieties. Like I said, some commas are unnecessary, some are necessary (grammatical ones) and all the rest are up to the writer. Remember that punctuation is basically just a way to mark pronunciation on written language. There's plenty of room for expression, but you can't break the rules.
Diana Hacker was clearly not a grammarian! She's very wrong about the things highlighted in red in post #18. I find it rather sad that people read her books. The Chicago Manual is quite reasonable about their advice, but you must remember who actually writes that manual, why, and who it's written for. Despite its apparent reputation in the US, it is not actually authoritative in any important way.
Unless you can point me to one, I simply don't know of a more authoritative American English reference than
Chicago. And Hacker's
Reference is highly regarded as a concise guide. That's why it's widely used.
But aside from our opinions about the merits of the two books, it still seems like we're agreeing. In the US, the entire approach to English is much more prescriptive than in the UK. To Americans, a grammar book is an instruction manual, a cookbook, a shop guide for repairing a 1978 VW bug. We're much more bound by our grammar rule books (I use "rule" intentionally) than the British tend to be. So British writers and editors are more likely than ours to leave out commas when - as you say - they're not really needed, rules be damned.
And I simply wasn't thinking about that difference when I recommended a comma before
but. (I don't think I was the only one who recommended it.)
Re whom by, whom for, and why: As far as I know, it was written by language academics for people who what to look things up for the purpose of informing. No more conspiracy theories, please! We're drowning in them over here!
Anyhow, by now I'm sure our poster has learned enough from all of us to decide where and whether to place the commas.
Thank you for your good thoughts!
cb
PS - Typing "UK" reminds me of a TV variety show that was popular here for decades,
The Tonight Show. When I was a kid, it had a host named Jack Parr. In his opening monologue one night, he said that he'd just returned from London. While he was away, someone in Hollywood phoned his studio and asked if he could speak with him.
His assistant said, "I'm afraid not. Mister Parr has gone to the United Kingdom."
The caller said, "I'm so sorry. Is it too late to send flowers?"