Thank you kindly for compiling all these sources of information into such a coherent guide. And more, for continuing to update! 
When I eventually found your S24 thread, it helped change my mind, having hypothesised (in a big Reddit post) about apparent matchmaker bias coming from MMR being off-set. I don’t think I can tag members here (?) so I’ll quote my new thread:
I borrowed the markup code for collapsible text sections from you too, which has helped make the lengthy breakdown of my statistical match analysis more approachable. Hopefully.
(1) I think I’ve been able to quantify evidence of apparent matchmaker bias there, against one role in particular. (Or at least show some very unlikely series of events that fits within my long term subjective experience of bias.)
(2) Also, I saw a weird blip, half way through, at the start of season 27, where the apparent bias went away: my 45% win rate flipped to 100% for 10 straight matches (across many different days). Anyone else see anything like that? It seemed it might have followed a period of several matches where my (medal) stats were particularly high (but mostly not winning). Like, perhaps it was a mechanism I triggered…? Or randomly allocated (or ~3% random series of events).
That also meant I got to verify that win streak bonuses are a thing (something I never expected to see for myself). I got up to 33 SR for the last 2 wins and 28 for a couple before that (which wasn’t quite unprecedented elsewhere).
(3) Call me ridiculous and desperate, but I was also testing, right from the start of the analysis, to see if QP stats/MMR might influence matchmaking bias. And while there’s no way to prove events weren’t circumstantial, throwing 10 (or so) QP games on support appeared to get that role unstuck in comp. With significantly more favourable (subjective) match balance ratings, that fell away, back to being more negative. Perhaps as the (un)biasing effect was diluted by comp matches (or, arguably, as the SR climbed slightly, but didn’t reach my probable skill limit).
(4) I saw some weird behaviour with my QP MMR (that I was estimating, at great length), after I un-threw my games on that role. Like, it was not at all just like climbing the MMR back up. The first game I stomped, with PotG, then the next was unwinnable and all over the place for a whole tranche of games after that. See graph [Fig.19] in the write-up.
So I wonder if that was a reflection of the MMR uncertainty value (you talk about above) suddenly going through the roof…? Or if it’s maybe a separate (secret) system for dealing with account sharing…?
You state that MMR changes more quickly when MMR uncertainty is higher. Which makes sense on a new account with accelerated SR changes. Maybe in QP, after a long time away. But would we really expect to see this in comp MMR? Assuming that SR really is just a projection of MMR, that would make for big gains/losses after, say, a year of not playing. Does that happen?
(5) I saw a whole bunch of other minor findings and trivialities. Like flex/ticket queues weren’t worst for match outcomes (maybe slightly positive, but on limited data points).
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Also, can I ask, is there any solid information about how the matchmaker tries to make pseudo-comps, by not having two players who mostly play the same character, on the same team (and now role)? It seems to work out an unlikely number of times, that I get a Rein player team mate, since I switched to maining Zarya (from main tank), for one example.
I’m sure Your Overwatch has talked about devs alluding to this, in the past… Maybe. And to counter-balancing uneven group sizes with (solo queue) players of a higher MMR (or other qualities). Though I couldn’t re-find that video, when I went looking for it last year.
Anyway, thanks again for the great post. And I understand if you want to stick to official sources and avoid speculating with me.