Regarding: "I wonder whether too many comments here are pedontic?" I am curious. Are you suggesting that many of those posting have put their feet in their mouths?

Don't be silly. Of course all Americans know what petrol is, unless they've been hiding under a rock, never watched TV, been to the movies, or read a newspaper. We likely wouldn't have been exposed to "petrol bomb", but I'm quite sure the average American could figure it out in short order. Honestly, it wouldn't surprise me if many people, American or not, don't know exactly what a Molotov cocktail is, especially those under thirty.

Wikipedia notwithstanding, most of the dictionaries and other sources I have checked say that the word itself is the backronym. Some suggest that either the word or the phrase could both be considered the backronym. And, just to play devil's advocate here, in cases like "tip", with a false etymology, does the etymology really matter? One could make a case for saying that "tip" is an acronym for "to insure promptness" regardless of its etymology. It certainly can be "...formed from the initial letters..."

Um, yes it is. A backronym is created when a phrase is written after the fact to align with an already existing word. The word is the etymology for the phrase, not the other way around. I agree that "to insure promptness" is not the origin of the word tip, but since the phrase has been created to match the already existing word, well, that's the very definition of backronism.

Goofy, perhaps tip is not an acronym, but definitely a backronym. By the way, I've also heard it as "to insure perfection".

Yes, it's a complete sentence. It's an imperative, urging you to, perhaps, participate in matters relevant to your local community and/or stimulate your local economy. I don't know your context, but I imagine it's something like "Don't travel to the Riviera; go to your local beach. Don't buy cheap foreign knock-offs. Forget the Louvre; go to a nearby museum. Eat at your local restaurants. Join the PTA. Live local."

Both are grammatically correct. Both mean pretty much the same thing. If anything, there might be a small difference of emphasis:

"When are you working?"
"I have to work tomorrow."

"What are you doing tomorrow?"
"Tommorow, I have to work."

Even so, you could just as easily switch the two answers to the questions above and still be equally correct.

In English, well, in all languages, there are always many ways to say the same thing. This should come as no surprise. Life would be pretty boring if this weren't true.

This is no big deal, really. It works the same for most other adverbs:
"Gingerly, he tiptoed through the room"
"He tiptoed through the room gingerly, "

Brus, I haven't spent as much time as you in the UK, but numerous sources confirm that collective nouns, specifically in the UK, are often treated as plural, even when their construction is obviously singular. "IBM are...", "The Parliament are...", even "The corporation are..." This applies when an organization can be thought of as a group of individuals. I have also heard it frequently in the UK media. And DA wood, "The waters of the Atlantic Ocean became his final resting place..." illustrates nothing. "Became" is the same for singular and plural cases. In your example, "waters" is plural. You would never say "the waters of the Altantic is deep"; it's "the waters of the Atlantic are deep". Even clearer, "the Atlantic's waters are deep."

D A Wood, "headquarters" is considered to be a plural noun with both plural and singular construction considered correct; but the plural construction is more common. I wouldn't say it's a collective noun (compare water and waters. both may seem "collective" but clearly waters is plural). Rather, I'd say it's more like pants or scissors, a singular entity today, whose etymology indicates an item originally conceived of as plural. Usually, as mentioned in several sources, the singular is reserved for cases when headquarters refers to authority rather than physical location, as in "Headquarters is sending us to the front lines". While not necessarily wrong, may I suggest that some of your examples use "is" by mismatching the verb case to the adjectival clause? "The headquarters of the British Commonwealth is in London, England." may sound correct because "is" is incorrectly associated with the adjacent "British Commonwealth" instead of "headquarters". Just a thought. Of course, in the UK (but not in the USA) one would more likely hear "The British Commonwealth are..." In the UK, entities like comanies, organizations, etc., are treated as plural even when they seem singular grammatically .

Daviderattigan, you said that "There is nothing linguistically inferior about a language form simply because it isn't ... acceptable in particular social contexts."

Actually, if a particular manner of speech is unacceptable in a particular social context, then wouldn't that be the very definition of "linguistically inferior"?

Yes, trained linguists avoid value judgements, but this is a matter of scientific objectivity. Linguistics experts may avoid such value judgements but others certainly don't. It would be irresponsible to tell someone that whatever they say is just fine with everyone when that's not the opinion of the world at large. We are all judged by how we speak all the time. The world is not the linguistic Shangri-la you are suggesting. Also, making a value judgement is not the same thing as being denigrating. Yes, "standard English" is far more vague a concept than many realize, nor is its use dictated for any particular forum. I would agree that different styles are appropriate for different environments. I would also agree that it's wrong to belittle someone for their speech patterns. But, face it, certain speech patterns are not considered acceptable to everyone everywhere. Fostering an understanding of this will only aid others in communicating effectively.