Origins of the Volitional Form

Question

I was doing some reading, and I read that while the volitional form can be explained as coming from the 未然形 for う-verbs, it cannot be explained for る-verbs. (I'm sure I should be using better terminology here, like 五段, but the correct words escape me.) Is this true? Can the usage of よ in the volitional form not be explained?

Answer 1

〔一段活用・二段活用の動詞に推量の助動詞「む」を伴ったもの，例えば，「見む」「受けむ」などは，中世末期までに「みう」「うけう」から「みょう」「うきょう」の形に変‌​化していたが，そこから，動詞未然形「み」「うけ」と助動詞「よう」とが分かれて，助動詞「よう」が生ずるに至った。現代語のように，五段活用の動詞には「う」が，その他の‌​活用の動詞には「よう」が付くというように，接続のしかたを補い合うような用法が一般的になるのは近世江戸語以降のことである〕
― 大辞林 entry for よう

Combinations of monograde (一段) and bigrade (二段) verbs with the conjecture auxiliary -mu, for example mi-mu and uke-mu, had transformed from mi-u and uke-u to m-you and uk-you by the end of the Middle Ages; however, at that point, they were reanalyzed as the imperfective forms (未然形) mi- and uke- plus the auxiliary -you ― and thus, the auxiliary -you was born into existence. The notion of complementary auxiliaries, such as in modern Japanese, where -u attaches to quintigrade verbs and -you attaches to other verb classes, did not become predominant until the modern Edo dialect. (Translation mine.)

Whether 大辞林 is to be believed or not, I'm not sure, but it sounds quite plausible to me. Classical Japanese: A Grammar by Haruo Shirane gives a similar explanation:

"In the Heian period, mu also appeared as the nasalized sound-change n. Beginning in the Kamakura period, this n changed to u (う), which is how it appears in modern Japanese. Thus, ika-mu (行{い}かむ, I will go) becomes ika-n (行{い}かん) and then ikō (行{い}かう). This u also combined with the preceding verb stem to become yō (よう), as in modern Japanese tabeyō (たべよう, let's eat)."
― Shirane, H. (2005). Classical Japanese: A Grammar (p. 99). New York, NY: Columbia University Press.