This is not exactly an answer to your question.
う originated from む, I think there should be no doubt. So 泳がむ→泳がう→泳ごう, the first form is used in old Japanese, the second form is used before World War II.
I think the problem is actually not whether う is a real particle, suffix or auxiliary verb, but whether 未然形 itself exists, or if the name is reasonable.
Personally, I believe the 未然形 cannot exist as a standalone form. In fact, the criteria used to classify the so-called “活用形” (namely 未然形, 連用形, 終止形, 連体形, 已然形, 命令形) are controversial.
連用形, 終止形, 連体形 are named after their functions. 未然形, 已然形 and 命令形 are named after their shapes. The problem is that verbs often have more than one 連用形/終止形/連体形's and their 未然形/已然形/命令形/連用形 may be the same.
You can see the chaos in this entry. り
り
［助動］［ら｜り｜り｜る｜れ｜れ］《四段・サ変動詞の連用形に「あり」の付いた語、例えば「行きあり」「しあり」の音変化形「行けり」「せり」の「り」から》四段動詞の已然形、サ変動詞の未然形に付く。ただし、上代では四段動詞には命令形に付く。
The rules in fact are unbelievably simple: 連用形(-i)+あり(-ari) becomes (-eri), that's all.
The only situations involving 未然形 that I can recall are:
ク語法、り（助動詞）、
られる（受身）、させる（使役）、
む（助動詞）、う（助動詞）、
ず（否定）、ぬ（否定）、ない（否定）、ざり
まい（否定推量）、まじ、…
べし（推量）、べき、...
It seems that ク語法、り（助動詞） all look like the result of assimilation of two syllables. ず（否定）、ぬ（否定）、ない（否定） share the same origin. The form used before る、す、まい, べし, etc. is not stable, there are often exceptions.
In fact, I have never seen 未然形 used without a suffix, nor is it separable. That suggests that 未然形 along with the suffix might be viewed as an inflected form, or a derived verb.
But there are many adjectives ending with the vowel /a/ or /ashi/ sharing the same stem with a verb, which makes me think that the vowel /a/ might have been widely used to express an kind of appearance. (maybe = 様{さま})
あか (said to be related to 明け{あけ} )
暖かい{あたたかい} -ai (said to be related to 熱い{あつい} )
明らか -aka
いやらしい -rashi
願わしい -ashii
望ましい -ashii
It seems that /a/ is often used to fill the gap between two consonants. As modern Japanese people tend to perceive the unstressed, reduce vowel (/ə/ schwa ) as /a/, is there any possibility that these /a/s actually reflected the old neutral vowel /ə/?
oːsuffix a single morpheme in modern Japanese.