Basically the Mullahs have an endless list of identical potential leaders. It's like a Hydra: killing one means the next one will grow right back. Therefore the Americans or Russians can't have any interest in this. It changes nothing for them.I'd rather look at:

First suspect: Who is next in line in Iran?

Why would one fly in fog like this: It looks like silent hill

Second suspect: Raisi just wanted to escape. Fog like this and it took them 6 hours to locate the helicopter? Also it's in the north west (=close to the Caspian sea). The guy wants to retire in a nice seaside homestead with nobody searching for him.

leaning toward an internal coup, or assassination from within his own country. I heard his own people were actually really happy & celebrating

Israel, Iran, Azerbaijan...