Hard to be conclusive with that video. Iran's own state media says the engine was on fire. Not necessarily contradicted by the video. https://en.irna.ir/news/83625750/147-Iranians-32-foreigners-...
Regarding the missile theory: if it was a missle, it was probably accidential. The majority of passengers on board were Iranian citizens.
Unless it was a non-Iranian missile, or a rogue actor.
(At this point, it's all speculation)
* Precisely for reason above
From a suggested twitter video. No affirmative source yet.
It's not hard to imagine the conversation would involve like:-
PILOT: One of the engines has failed catastrophically, we need to make an emergency landing, can you clear the runway back in Tehran or is there another airport we can go to?
They would have been rather occupied with the first two.