Just to add to what you said but regarding this particular incident, if the player doesn't go down here then it gets brushed away, the referee missed the incident completely and the assistant sees this from behind, so if he doesn't go down the assistant doesn't see the foul therefore the GK gets away with violent conduct, the reaction itself may be down to surprise because I'm sure if you were standing beside someone waiting for the ball coming in then instead get an elbow to the face you'd be a little surprised.
(not condoning players acting injured however sometimes going down highlights the foul that wouldn't otherwise be given, I'm sure people have been in a position where you've hesitated to give something because the player has stayed on his/her feet)
I certainly have. 2 weeks ago a gave a pen when the striker stayed on hisnfeet but there was clear contact and it caused the striker to stumble when he was about to shoot. I gave the pen (I usually avoid giving pens if the attacker doesn't go down unless it's really blatant like most I'd imagine if we're honest). Attacking team thought it was a brilliant decision and praised my bravery. Away team thought it was a shocker and threatened to report me. This was after their striker had received a 2nd yellow 2 mins earlier for deliberately scoring with his hand. Turns out he wasn't registered anyway!
Having given the penalty, I knew I was 100% correct to do so but I also knew that the defending team will go mad because normally players do have to go down (I'd have done the same probably). I'm also not sure if I'd give it again,I think I would but truth is when a striker stays on his feet we often look for a way to not give it at least some of the time. It's a tough one for any ref