Answer for old words: buncha dead white dudes said so
New words: we took them from Black teen girls
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No idea. I’ve been trying not to look to deeply into Japanese etymology because I agree that that it’s fascinating, and my review pile would never see me again.
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米国 (currently read as “bei guo” in modern Mandarin) is also the term for America in Mandarin Chinese, so I honestly couldn’t say for certain whether Japanese had the ateji or the abbreviated form first.
As far as I know, it’s 美国 in Mandarin at the moment (měi guó). Mandarin and Japanese use strikingly similar abbreviations for certain other nations and continents though, like how 亜 (or an equivalent) appears in the name for Asia in each language, or 英国 is used for England/Britain and 印度 for India. Romaji are probably still more common in Japanese though.
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We studied this in my Japanese linguistics class. The importation of the sounds came with somewhat strict Japanese-ification (sorry I can’t remember the official term). Sounds that ended in a consonant had a vowel added to them- typically u or i. There are other changes as well. My friend in the class, a student from Hong Kong was able to identify many of the reading origins once we learned how they were changed.
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Why is the thread called “Were there kunyomi for compound words in the past?” by the way?
There are plenty of compound words that are fully kunyomi, now and then (and before the concept of “kunyomi” was a thing, because at that point it was just all Japanese and nothing else).
The question seems to be about if there were words that filled the roles of currently widely used onyomi words, but I don’t really see how compounds play any role in the discussion.
I get that there are some examples of kanji compounds which have onyomi in the first post, but “words that are kanji compounds with onyomi readings” is just one part of “all compound words.”
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Geh. I had a feeling that might have been the case, but it’s been a depressingly long time since I last studied Chinese…
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It’s OK, you were really close. To be honest, I’m still wondering how Japanese and Chinese chose the transliterations for ‘America’. Maybe they’re based on the second syllable? That doesn’t really explain 米国 though, because it’s “mi3” in Mandarin, but ‘bei’ in Japanese. 
EDIT: Oh, apparently ‘mai’ is another possible reading for the kanji. I wonder if something happened during the abbreviation process.
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I believe it’s the beginner learner advice of “onyomi are used for simple/single morpheme words in Japanese, but kunyomi are often used in compound words.” We don’t need to question simple words on the basis that they are unlikely to use the kunyomi.
Feel free to correct me if I’ve inferred wrongly, OP or others.
I’m not sure if anything can really be said about compound words as a whole. For instance, any time you get two verbs becoming one, that’s a compound word, like 聞き取る, or 飛び出す, or めしあがる. There are effectively an infinite number of those, because some can be used on the fly to connect with any verb, and they’re basically all kunyomi.
But I guess I’m just being picky about the title wording.
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nooo, now I want to understand as well!! why から and not そら ??
空 as から means vacant, empty
空 as そら means sky
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In addition to @zyoeru’s point, you can’t just pick and choose readings as you like. からぐるま is simply how the word is read. 
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I have to use a vocabulary book provided by the city for one of the classes I teach. In it, the word “empty” was defined as 空の, and the example sentence made it clear that it was meant to be から.
This didn’t stop students from writing things like “The empty color was beautiful.” Where they thought they were writing そらのいろ.
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@ollylove If you need a related word for help… ever heard 空っぽ（からっぽ）? Same root.
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or 空手, which has even made it to English 
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Good point! I forgot about that one.
(Apparently it used to be written as 唐手 though. I was so shocked when I found out.)
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Or 空オケ
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I was told this in karate class I think and I always find it amusing when that kind of anecdote actually turns out to be correct!
I remember watching a let’s play where the player accidentally read 空のスロット (empty slot) as そらのスロット (sky slot)
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I’ll also add that I agree that comparisons with Latin can make these kinds of things more intuitive! It’s not 100% but European languages tend to use Latin (or Greek) for more fancy-sounding words, same as Japanese tends to do with Chinese words.
So you’ll rarely call simple objects by their Latin names (like aqua), but you’ll find it in words like aqueduct, aquarium or aqua gymnastics.
That doesn’t mean you’ll never find “water” in scientific compound words like water pressure or whatever. Just like you’ll still find kanji compounds that use the kun’yomi.
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