If it was the Musad and it was poison, then they lost an agent who was obviously well placed. It seems unlikely that they’d sacrifice an agent just to kill one man. The typical means of killing someone in Iran is for two men on a motor bike to come up beside the victim while he’s stopped at a traffic light. Then, the rider smacks a sticky bomb on the window, and they speed away. Four or five seconds later, boom! I’m thinking if it was the Musad, they wouldn’t sacrifice an agent to kill just one man. Then, the Musad would have to get him out of the country. Nope. Unlikely.
The Russians, on the other hand, often use poison to kill people. Several times in the last twenty years, they’ve flubbed it terribly, though. Poison just isn’t the surest way to kill someone.
Possible the agent’s cover was about to blow. Now it certainly has. Good points, though, whatever the truth happens to be.
But the Russians have no motive to go around killing Iranian rocket scientists, unless they want to blame it on Israel, in which case they would have used the usual Israeli MO, as you said. Perhaps Israel wanted him dead for obvious reasons, but used a Russian MO just to muddy the waters. But they’ll probably blame Israel anyway, because that’s how Iran rolls. So the waters remain un-muddied.
Bad dates
https://youtu.be/DrfKxqZyPWU?t=131