And each derived from context where it is ambiguous.
No, there's nothing ambiguous about it. The word you're searching in vain for is polysemy. Means that a word has multiple meanings. (Related to semantics, the study of meaning.) It's a common feature of words in many languages, particularly English. And there frequently is no "most common meaning". There's nothing ambiguous about, e.g., using "can" as a verb vs. a noun, because it's impossible to confuse them in context.
If you really think that "kills one in ten" is the most common meaning of "decimate", though, then it should be easy for you to find some examples of people using it in that sense. Real examples of real people using it that sense, not something you made up. I contend that it hasn't been used to mean that for at least a century. If you just google the word itself, you'll mostly get definitions and people discussing the word. I'm looking for examples of them using the word in what you seem to be claiming is the most common sense. I've checked Google books and Google news, and I can't find a single example. Just find me one, and I'll be impressed (though one example will hardly prove your point).
In this case, there's no ambiguity. It's just wrong. People type "alot" frequently and have no idea what "fewer" means.
Define "wrong". If it's wrong, why does it appear in every dictionary? Including the OED? (Even if they don't list it first, for whatever reason.) Dictionaries don't list things that are "just wrong". They don't list "alot", for example. So, why should I trust you, some random person on the Internet, over actual lexicographers and linguists, who all disagree with you?
(And "alot" is a spelling mistake, which is a whole different kettle of fish. Standardized spelling is much more recent than words having meaning. Words have had meanings for probably tens of thousands of years, if not more. Standardized spelling is only a couple of centuries old. There's a lot of reasons that spelling doesn't shift the way meanings do, but that's getting off topic, and it's really not my field. Bottom line: bad analogy.)
As for "fewer", pretty much everyone knows exactly what it means. The word that cause confusion is "less". Some idiots believe that because "fewer" is only used with countable nouns, that that means that "less" can only be used with non-countable nouns. That would make sense if the words were designed to complement each other, but they're weren't designed, and they're completely unrelated. Less is much older and has been used with both countable and uncountable nouns for over a thousand years. Claiming that it's "wrong" to say "10 items or less" is simple linguistic ignorance. Saying "10 items or fewer" is also acceptable, of course, but more stilted.
Any more dumb linguist superstitions you want to throw at me? No splitting infinitives? No ending sentences with prepositions?