I agree - emphatically! - that eyewitness testimony is a lot less reliable than most people believe. I mostly only brought the witnesses up in my discussion because I thought the jury dismissed them for bad reasons, instead of a general policy of "eyewitnesses are unreliable". (In retrospect, I could have been a lot clearer on this point.)

I agree that the knife being unique would have made things a lot more clear-cut, but disagree about the implications.

If no-one is deliberately trying to frame the accused, the odds of the real killer happening to use the same brand of knife as the one he favors are very low. (What fraction of knives* available to potential suspects are of that exact type? One in a hundred, maybe? If we assume no frame-up or suicide and start with your prior probability of 10% then a naive Bayesian update and a factor of 100 moves that to >90% even without other evidence**.)

If he is actively being framed . . . that's not overwhelmingly implausible, since it's not a secret what kind of knife he uses, and the real killer would be highly motivated to shift blame. However, the idea that he'd have lost his knife, by coincidence, at the same time that someone was using an exact duplicate to frame him (and then couldn't find it afterwards, even though it would be decisive for his defense) . . . strains credulity. I'm less sure about how to quantify the possibility a real killer took his knife without him knowing, got into the victim's apartment, and performed the kill all while the accused was out at the movies; but I feel pretty confident the accused's knife was the murder weapon.

*I'm ignoring the effects of the murder weapon being a knife at all because they're surprisingly weak. The accused owns a knife and favors using it, but so would many alternative suspects; and the accused cohabiting with the victim implies he also has easy access to many alternative methods - poison, arranging an accident - that Hypothetical Killer X wouldn't.

**Full disclosure, I didn't actually perform the calculation until I started writing this post; I admit to being surprised by how little a factor of ~100 changes a ~10% prior probability, though I still feel it's a stronger effect than you're accounting for, and for that matter think your base rates are too low to start with (the fight wasn't just a fight, it was the culmination of years of persistent abuse).

I agree that the protagonist having ideological or personal reasons to make the case turn out this way is much more likely than him having been successfully bribed or threatened; aside from anything else, the accused doesn't seem terribly wealthy or well-connected.

I also agree with your analysis of the racist juror's emotional state as presented, though I continue to think it's slightly suspicious that things happened to break that conveniently (the Doylist explanation is of course that the director wanted the bigot to come off as weak and/or needed things to wrap up satisfyingly inside a two-hour runtime, but I'm an incorrigible Watsonian.)

Literally everything the racist juror does in the back half of the movie is weird and suspicious. It's strange that he expects people to be convinced by his bigoted tirade; it's also strangely convenient that he's willing to vote not guilty by the end even though he A) hasn't changed his mind and B) knows a hung jury would probably eventually lead to the death of the accused, which he wants.

I don't think it's likely, but I'd put maybe a ~1% probability on . . .

. . . him being in league with the protagonist, and them running a two-man con on the other ten jurors to get the unanimous verdict they want.

1. The existence of other knives of the same kind as the murder weapon is almost perfectly useless as evidence. The fact that the knife used was identical to the one the accused owned, and was used to kill so close to when the defendant's knife (supposedly) went missing, is still too much of a coincidence to ignore. The only way it would realistically be a different knife is if someone was actively trying to frame the defendant, and arranged for his knife to be lost at the same time; and if they could do both of those things, it makes more sense for Hypothetical Secret Mastermind X to just stab the victim with the accused's actual knife. (This means Juror 8's illegal purchase of an identical knife in the name of justice was epistemically pointless, and only served to muddy the waters; I'm oddly enamored by the probably-accidental pro-Lawful-Good thematic implications.)
2. The old man's testimony is suspect for more reasons than the jurors notice. The lack of fingerprints on the murder weapon suggests the culprit wiped it off first, but the old man claims the culprit ran off immediately after the body hit the floor. However, this aligns with the other reason to consider him unreliable, which is him (allegedly) managing to move quickly enough to see the accused leave the scene; it seems pretty plausible that he got the timing wrong but everything else right.
3. The paramedic juror's claim that the knife was used incorrectly - that it's the kind of knife made to stab up through the gut instead of down through the ribs - doesn't exonerate the defendant, and might actually incriminate him. It's a fact about the knife, not the user; if anything, a young man might be more likely than the average assailant to wield his weapon wrong.
4. The other witness turning out to (probably) habitually wear glasses doesn't necessarily make her testimony invalid. She could be farsighted, could need reading glasses, or could just habitually wear them to seem intelligent or as a fashion statement. All of these explanations seem more likely than a - by all accounts, scarily competent - prosecutor putting her on the stand without checking she could actually see the murder. (None of the jurors consider requesting additional testimony on this topic, even though it's both easy to check and the point which ends up deciding the final verdict.)

The accused is very likely to have committed the murder.

The protagonist probably has some kind of agenda: either he takes issue with capital punishment, knows the defendant personally, strongly dislikes the carceral justice system, is being bribed, or is trying to arrange acquittal for a guilty party just to see if he can.

I still think a case can be made for the existence of reasonable doubt.

You consider the possibility it was a suicide.

If I knew for a fact the defendant was innocent, most of my probability mass would be on some variation of the following sequence of events.

• The 'victim' has his (injury-free) altercation with the accused. This rattles the accused to the point that he forgets to take his knife with him when he leaves for the movies; he falsely assumes that it "fell out of his pocket".
• The 'victim' is also rattled, and decides to commit suicide. (Possible motivations: realizing that he can no longer reliably win a fight against the target of his abuse and wanting to quit while he's ahead, feeling regret about his treatment of the accused, being angry at the accused and wanting to die in such a way that the accused ends up accused.)
• The 'victim' stabs himself in the chest, and not through the gut, in an attempt to end his life as quickly, painlessly, and dramatically as possible. Possibly he shouts "I'm going to kill you!" as he does this, either out of genuine self-loathing or an attempt to implicate the accused; possibly he makes a point of staggering around near an open window with a knife sticking out of his chest before collapsing; alternatively, the witness testimonies may just be mistaken and/or falsified for reasons discussed in the film.
• The accused returns home to find a dead body and two policemen. Between the lingering effects of earlier events, the presence of the victim's corpse, his current predicament and (quite plausibly) some mind-altering substances he chooses not to admit to using in the subsequent trial . . . the accused finds himself unable to provide satisfactory answers when the police ask for the titles and lead actors of the movies he watched. (He may or may not be able to recall other details about these movies: but either he doesn't think to volunteer this information and the police don't ask for it, or the police choose not to record inconvenient facts in an attempt to close the case cleanly while technically telling the truth.)

This hypothesis makes sense of the paramedic's claim about the type of knife, makes sense of the silent evidence of neither the accused nor the corpse having any injuries mentioned aside from the single stab wound (a person comfortable with violence yells an explicit verbal warning at another person comfortable with violence, and then stabs him to death, but there's no sign of a struggle?), and is supported by base rates (suicide is significantly more common than homicide in first-world nations).

. . . to be clear, I'd still say murder is much more likely, but I consider the above possibility just possible enough to be conflicted about the reasonableness of reasonable doubt in this case.