Now to SBU’s new theory. While something like this (the terrorists planning to destroy a Russian plane to provide a casus belli) is possible in principle, I’m worried by the fact that they already claim this hypothesis to be the truth, even though in their video -

Терористи та військові найманці цинічно планували теракт проти цивільного літака Аерофлоту

there is no evidence of this per se, only evidence that it was possible (because the planes’ routes intersected). Possibility does not translate to probability.
Not to mention that acc. to SBU’s own intercepts the terrorists thought that they downed an Ukrainian military plane (and indeed, that’s how their sources bragged about it to the world in the first hour after the tragedy). This seems simply to contradict that the plane was downed with any such intent.