So let's return to my assertion that often times Mandarin is studied as the standard Song vernacular and has roots that date back much further.
Norman explicitly calls out Mandarin as the predominant language of the Song Dynasty and dating to the Tang.
"The first type includes the Jianghuai dialects and Southwest Mandarin and may be traced to the southern type of Mandarin which flourished during the Song-Yuan Period. The second type includes the rest and may be traced to the northern type of Mandarin of earlier times.... It is also shown that Mandarin as a dialect group was already in existence in the eighth or ninth century, that is, in the middle of the Tang dynasty." [Norman, "Some Thoughts on the Early Development of Mandarin", In Memory of Mantaro Hashimoto, 1997, pg. 21-28]
But I believe you are also missing my point in linking to my other comment.
Everything I've listed there indeed has a pre-Song history (most from Sui/Tang, some from the Northern/Southern Dynasties, 是 as a copula Mair will argue comes even from the Warring States period). They are also why some studies of Mandarin will take Mandarin to have started in the Tang (or earlier!). But taken as an ensemble they are the DNA of what structurally (rather than just phonologically) distinguishes Mandarin from the other Sinitic families and the fact that they are ubiquitous across Song vernacular from all parts of China make it pretty overwhelming that the standard Song vernacular is structurally a Mandarin language, rather than any other Sinitic family. This is a harder case to make pre-Song (but still can be and has been made), but it's very clear by the Song.
yorwba's question is a very reasonable one precisely because the things you've raised are not uniform across all Sinitic families. 那 is not the usual demonstrative in a lot of these families (rather we have in various cases including Min and Wu, where we have demonstratives such as 彼 or 許 alongside some varieties of 爾). Likewise 是 is not the usual copula in some Sinitic families (e.g. Yue where the usual copula is 係). Another hallmark is pronouns where 你 and especially 他 are not the usual second and third person pronouns (the third person is especially different, where it is 佢 in Yue or other various versions of 渠 or 伊 in other families).
This is what we mean when we say that the different Sinitic families differ in grammar and structure rather than just phonology, and why it is possible on the basis of this grammar and structure alone to identify something as Mandarin.
More to the point we can and do analyze the grammar and structure of various medieval Sinitic families separate from their phonology. Open up any post-Tang period-specific textbook and that is exactly how it is laid out (usually the first chapter is phonology and then the rest, vast majority of the book is grammar and structure). We can do this as soon as we have a flourishing vernacular body of writing, which starts roughly in the Tang (with fits and starts before that starting in the Three Kingdoms), but really hits its stride in the Song. The only reason we have to resort to phonological-specific measures for OC is because it is unclear whether we have any vernacular writing from that time period and thus we have to use phonological hints to try to deduce to what extent a diglossia existed.
And what of the particulars of my other comment? My assertions about the pronouns, demonstratives, etc? Well you can find those in any intro Song vernacular textbook. For example I happen to have 宋代語言研究 (2001) by 李文澤 on my bookshelf at the moment and I can back up everything I wrote off the cuff there in his 語法編 chapter starting on page 214. You could also cross-check against the 近代汉语概论 by 袁宾 (which regretfully I do not have on-hand at the moment so I can't give you specific page citations). It's also blatantly obvious as soon as you start reading various Song Buddhist dialogues, 理學 dialogues (not their Classical Chinese prose works but their vernacular dialogue collections), 話本, particularly vernacular 宋詞, or any other Song vernacular works that what I'm saying there is borne out.
> the phonological and comparative regional linguistic studies have been the single main instrument that has made the reconstruction of Old (Ancient) and Middle Chinese languages possible. Which also includes the understanding of the grammar of Old Chinese and which was vastly different even from that of Middle Chinese.
First a correction: The knowledge we have of OC as a living language is extremely limited. Hence it is debatable whether we have any good understanding of the grammar of OC (the Chinese side of scholarship usually rejects this and says we do have a good understanding of OC through Zhou texts) in the same we have for MC. What Wikipedia often calls OC is debated whether it was actually ever spoken by anyone, or is just an artifact of a Classical Chinese that was in fact never a living language. Mair makes this argument forcefully in his seminal 1994 article "Buddhism and the Rise of Written Vernacular in East Asia:"
> Linguistic data that LS [Literary Sinitic, i.e. Old Chinese as represented by Warring States and Sprint-Autumn texts] and VS [Vernacular Sinitic, i.e. the true living, spoken language] have been distinct systems as far back as they can be traced. This is certainly true from the Warring States period on, but I suspect that eventually we will be able to demonstrate conclusively that LS, starting with its earliest stage in the oracle shell and bone inscriptions (around 1200 B.C.E.), was so obviously abbreviated and so replete with obligatory nonvernacular conventions used only in writing that it never came close to reflecting any contemporary living variety of Sinitic speech. [Mair, "Buddhism and the Rise of the Written Vernacular in East Asia," 1994]
Now I'm personally not sure how much I believe that (and e.g. the Chinese side of scholarship generally rejects this argument), but I bring it up to underscore again the reason we cannot do a phonologically independent analysis of living (at the time) Sinitic varieties before the Tang era (well kind of, we have some vernacular before then, but certainly by the Han) is because it's not clear we have any direct written evidence of them, whereas by the Song it's extremely obvious that we have a fairly true-to-life structural reflection in writing of how people actually spoke, and hence we do not need to rely solely, or even mostly, on phonological analyses.