Very good points.i'll throw it out there if FSB were behind this, they could have done a better job of making it look like Ukraine involvement. as it is, there's just opportunistic use to blame Ukraine.
here's another interesting side, what if the Kremlin are actually convincing themselves its Ukraine, indicating disfunction in the security services and leadership, taking their eye off other internal conflicts.
And if Putin claims it is Ukraine, a sizeable proportion of his own security forces (if the not the bally lot of them) will go along with him, for the simple reason they prefer staying alive.