I would not be surprised if all it takes to override the keys is one person who knows the technical aspects and a bit of time, or an engineer and more time.There were other equally plausible explanations at the time, and either way to this day this support still isn't proven beyond any reasonable doubt. There is enough information to conclude with high probability it happened (and likely happens to this day), but nothing conclusive.
But if I support position X, why would I stop defending it? Especially when after I do say that most people in conversation suddenly starts to think absolutely everything bad that was written about Russia in that conflict is true and i'm confirming it.
Simply because that's not how Russian forces were operating there in general. It breaks the pattern in big way. Ammunition wasn't "disappearing", it was delivered to specific groups and locations that needed it; i don't see why that particular direction needed something as big as BUK without rest of the package, and why would anyone turn control over it (or at least target selection) to local Ukrainians.
Social media, same place videos of BUK came from.
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Those were claimed by both sides (rebel and Ukrainian) to be captured without keys (which are supposedly unique for each, and BUKs cannot launch missiles without them). Rebels wanted to circumvent it but supposedly lacked expertise in that regard - it's not like BUK schematics are freely available on Internet.