you seem to want to argue that people who've sworn an oath to protect the USA, or who advise the US on how to best beat Russia (e.g. RAND) shouldn't be assumed to be biased in favour of the USA
Not at all Mendel. I'm saying the experts cited cannot be automatically deemed biased, propagandists or incompetent (like you seem to be arguing) just because they disagree with your pro-Russian narrative or have previously been involved with the US government in various capacities. Plus I made a second point which need not be repeated.
and also reverse the burden of evidence
The evidence that their speculation is not to be naively equated with any other Tom, Dick and Harry guesser is the overall soundness and dispassionate manner of their argument and analysis as well as their professional credentials and earlier analyses. But like I clearly said, even that wouldn't prove they're right. Empirical evidence pointing to Russia would.
how do you distinguish a "superior speculation" from a lucky guess?
By meta-analyzing, after the fact, the soundness, the detail and the rationale provided by these two 'analyses' (i.e. lucky guess and superior hypothesizing) and comparing which one is better supported by the facts. It's not rocket science.