Going off votes, it appears that there are other folks interested in this thread so I'll add an addendum here.
Let's begin with
> historical studies of phonological and tonal changes in Old (Ancient) and, later, Middle Chinese assertively point to emergence of various Chinese languages that we identify today as Wu, Yue, Mandarin etc to Late Middle Chinese (LMC) (with only exception being Min that had split off from the Old Chinese before Middle Chinese emerged)
I'm going to need to see citations from you in turn for this. In particular I'd be shocked to see any scholar in the last 50 years postulate that Wu split from LMC (again keeping in mind that "Middle Chinese" is basically more of an analytical shorthand than a real language).
To refresh my memory I decided to look at Chappell's introduction to Sinitic grammar: Synchronic and diachronic perspectives (2001), "Synchrony and Diachrony of Sinitic Languages: A Brief History of Chinese Dialects" (which conveniently is included in many course syllabi and hence the introduction can be found online in many places such as here: http://bartos.web.elte.hu/ma/chappell-dial-intro.pdf) and the situation is as crazy as I remember.
This is her summary of Xiang:
> The language of the Chu kingdom, an important dialect before the Jin dynasty was established in the third century CE, was spoken in the area of modern-day Hunan and Hubei. It appears not to have been mutually intelligible with the court language of the Zhou, Xià 夏, the goal of reconstruction for Archaic Chinese. You (1992) regards Old Chu as the basis for Old Xiang which probably split off from Medieval Chinese some time prior to the Tang dynasty. [pg. 10]
This is her summary of Northern Gan:
> Northern Gan was probably formed during the period between the end of the Eastern Han and the beginning of the Tang dynasty (third to seventh centuries CE) [pg. 11]
This is her summary of Wu and Min:
> The less contentious standpoint would be to posit a common ancestor for Wu and Min, formed by the first century CE, from which both split off. [pg. 12]
This is her summary of Mandarin (which she uses interchangeably with 北方話)
> The concept of běifānghuà 北方話 ‘northern speech’ is first mentioned in the dialect work by Guo Pu 郭璞 known as Fāngyán Zhù 方言注 (Commentary on the Fāngyán) and compiled during the Eastern Jin dynasty (317-420 CE). Certain dialect words, listed in earlier works such as the first century Fāngyán 方言(Dialects) by Yang Xiong, are given in this later work by Guo Pu as the common term in northern speech (You 1992: 94). This is interesting in that it suggests some unification of the northern dialects of Chinese had already taken place by this time - the period of Early Medieval Chinese in the fourth and fifth centuries CE. [pg. 10]
This is her summary of Hakka:
> In the first view, espoused by Lo (1933), Hakka was already formed in the north in the area of present-day Henan province, before the first southwards migration took place in the fourth century CE following the fall of the Western Jin dynasty in 313... In the second more plausible view, Hakka evolved in the south in the period of the Song dynasty, specifically in southeastern Jiangxi and western Fujian, after migration had taken place [pg. 15 - 16]
This is her summary of Yue:
> The Yue dialects were thus formed in a language contact situation with non-Han peoples in a frontier area, first annexed as Chinese territory in the Qin dynasty (late 3rd century BCE), as outlined above. [pg. 16]
The language situation is both far older and less well-ordered than you are making it out to be. Also just to underscore how "Middle Chinese" is really just an analytical tool than a real language, note that Chappell also almost never talks of "Middle Chinese" other than the "Middle Chinese phonological system" and instead is careful to talk about "Medieval Chinese" instead.
Also profuse apologies to Sagart! I apparently have been misremembering his position and have been confusing it with authors he was arguing against!
But to sum it all up yes, I will stand by what you are asserting is a "fringe theory," but which ample evidence suggests is fairly mainstream, or as you put it
> Summing it up, so far you have presented an alternative timeline of the history of Mandarin that has had a supposedly continuous history of existence starting from (or even predating) early Middle Chinese.
The only contention is to what degree what we call Mandarin existed pre-Tang (as Chappell suggests, there is evidence, but it gets very fragmentary very quickly). But Norman provides clear evidence by the 8th century (which he hedges with 8th or 9th, but a lot of the texts that he is transitively citing are 8th century texts) there is clear evidence of a mature and distinct Mandarin family and there is fragmentary evidence that would push the beginnings of the Mandarin family even earlier (Norman believes no earlier than the beginning of the Tang, which I can accept, but again other authors will find glimmers of early Mandarin even earlier).
And I stand by my original comment to hker, which is that Mandarin has been the spoken standard (or different varieties have been simultaneous standards depending on specific times) for literate speakers across the Chinese empire for > 1000 years (certainly by the Northern Song).
P.S.
There is one clarification I'll make to Chappell's summary here:
> By the time of the Southern Song, early Mandarin extended only as far south as the middle and lower reaches of the Yangtze, apart from one small pocket where Northern speakers had crossed the Yangtze into the Northwest Xiang area after the Ānshǐ 安史 turmoil of the mid-Tang dynasty [pg. 11]
Apart from the implication here that Mandarin was distinct and already existed by the mid-Tang dynasty, the reason why I brought up the fact that 朱熹 is a Fujian native (and spent the overwhelming majority of his life there) is that he is a good demonstration that while Mandarin may not have been the first language of a lot of people in China during the Song, it was the lingua franca. Indeed while Fujian does lie outside the region Chappell is talking about, and 朱熹 does make reference to a distinction between proper speech and the local language, the fact that his vernacular writing is effectively the same as elsewhere in the empire is strong evidence for a Mandarin-based lingua franca among at least literate speakers.
And finally for a more authoritative source that the core of Mandarin vocabulary remains mostly monosyllabic than "look at Swadesh word lists", there's 论汉语的单音词 (2009) by 李如龙 who states in no uncertain terms:
> 越是常用的词 ,单音词所占比例越大。在500 个高频词中 ,单音词的总数比双音词多出一倍。一千个高频词之后双音词才大幅度增加 , 到 1500号 ,双音词才和单音词平分秋色 ,数量相当。