Is it really unique to Britain? It's the first definition on Wiktionary and it doesn't mention anything about it being regional. Is scheme really only negatively in America?
It might be used this way in other commonwealth countries. In the US we'd use plan, project (personal endeavor), or program (government endeavor) instead.
Scheme itself means the same thing but it's fallen out of use and the only times I've encountered it is when the speaker wants to distance themself from it: get rich quick scheme, hare-brained scheme, nefarious scheme, malicious scheme, etc.
Yes. The Wiktionary entry mentions this: "In the US, generally has devious connotations, while in the UK, frequently used as a neutral term for projects"
> I don't know how the word carries so substantially different moral leanings, but that's English.
There's a book published by some Brits in Oxford that very usefully researches the history of English words. They have found examples of all the meanings discussed here from the mid-to-late 17th and early 18th centuries, FWIW.
English doesn't need words with evil twins. English is much more cunning than that in that the same word can have totally opposite meanings. They tend to be more slang usage, but for example "shit". Without recreating the George Carlin bit, there's so many different meanings depending on how it is used.
Word meanings evolve many ways. Sometimes two word forms evolve independently and coincidentally to the same outcome. Sometimes a word develops from Latin or Greek, and then centuries later someone coins a different word from the same Latin or Greek. Sometimes people apply old words to new situations. Etc. If you look up the field of etymology, you can find many models of how words form.