Both reds fully justified. Assaulting the referee - gone.
Knocking the cards out of the hand (although I can't see what happens in the video) for the player to then have the audacity to pick it up, raise it above the head and show the ref the exit was a moment of madness and is unacceptable behaviour. I had the same situation in December, player walked.
There's no place for that sort of behaviour. You also have to bare in mind you can't hear what is being said to the referee, this may have contributed to the red card.
As for the why not put the red away... Think about it... He's just sent a player off, leaves it out as a deterrent... I'd have done the same until the situation calmed down! Confrontations are ugly and should be dealt with accordingly. Good on this ref!
As for multiple cards for dissent... Although yes there is a case, what do you think could have happened had he started to dish out the cautions? My guess is that the already simmering temperature would have reached boiling point! He's managed the situation well here.