As for your what-if's: that's what a jury is for. Determining matters-of-fact from presented evidence. They convicted. If they could (and did!) do that without

As for your what-if's: that's what a jury is for. Determining matters-of-fact from presented evidence. They convicted. If they could (and did!) do that without needing DNA evidence, the presence of someone else's trace DNA, or absence of the suspect's on one specific piece of evidence isn't likely to change that, even if it's the murder weapon. All evidence is imperfect, and I personally think we'll start to publicly realize that DNA evidence is just as malleable as eye-witness testimony. Just run more PCR iterations until some other DNA is amplified above the signal/noise floor! (If people knew the amount of analytical probability that goes into PCR/ matching DNA samples, they'd have a lot less faith in the technology, but that's another topic.) Same thing for manipulated/edited video and even deep-fakes. Which, isn't too say all evidence is crap and therefore no convictions are valid. It's why we put faith in the system and in our peers serving on petit-juries... and why it's up to a them and the judge to evaluate the evidence, and decide whether or not to convict... And why we don't have a computer run a PCR and automatically say: "Positive dna-sample: go directly to jail".