Thanks for sharing your thoughts and you'd be right about the inconsistencies.
For example, the 850CSi was a full-fledged product of BMW M with a "WBS" VIN prefix indicating as such as well as an S-series motor (S70B56), but sans the M-badge in the model designation. Of course, we now know that the reason can be partly attributed to the fact that M GmbH had an M8 in the works, a project that was ultimately scrapped, leaving the 850CSi as the highest-tiered 8 Series.
Of course, the M635CSi was a M-car through and through, where it is essentially the first generation M6 and actually sold as such in Japan and North America alongside the first M5.
The E30 two-door (incl. M3) of course, as you already mentioned, wasn't a "coupe" by definition at all. It was actually a two-door sedan (same as the E21 and 02 Series BMWs), hence "sports sedan". Naturally, then, it is easy to understand why the "C" suffix wouldn't be applied to the E30 two-door models, since they weren't coupes by definition.
This is a difference not a lot of people are aware of, and I had, at one time, attempted to explain it
here.
The E36 two-door is interesting, because, for the first time, the 3 Series two-door was being marketed and referred to as a coupé. But this is where the lines are kind of blurred (grey area, so to speak). Some contemporary literature continued to refer to/classify the E36 coupé as two-door sedan, or through indirect references, such as calling the E36 M3 Coupé a "sports sedan". I would argue that during the E36-E46 generations, the 3 Series Coupe was kind of both sedan and coupé...a sort of compact, sports, coupé-sedan thing. For all intents and purposes, the greenhouse & roofline of both E36 & E46 two-doors were fairly upright (i.e., not sloped) and the seating accommodated a five passengers (not 2+2). They very much continued on with the same formula as the E30 two-door.
But of course, as we all know, the E46 two-door would be marketed as a coupé with its model designations even gaining the "C" suffix and by now, colloquial usage of the word "coupé" evolved to basically mean any fixed-top car with two doors.
With the E90 generation, the seeds of change were finally planted.
The 3 Series coupé would become a true coupé by definition, with an entirely different exterior design, 2+2 seating, sloping coupé roofline, and a separate internal designation (E92 in this case)
In fact, there were early rumors that the E92 would be separated into its own model series (i.e., 4 Series).
Henceforth, the "C" designation in model names became redundant and irrelevant. BMW decided soon after that models that would be marketed as coupés would be separated into their own Series (i.e., 2, 4, 6, 8), rendering the "C" suffix, largely obsolete.