From my experience with firefighting..
The passengers don't appear to have felt much radiant heat. The flames are a good distance from the cabin in this respect. The wing sheilds the cabin from the greater fireball under the wing.
Perhaps the passengers were fairly chilled because they presumed there was no risk as after the turn back, they had been airborn for so long.
Also there was no communally felt bump or bang nor smoke in the cabin.
No chorus of arhhhh to get the adrenaline going.
So only those near the windows on one side would initially be aware of the scale of the fire.
So prolonged flight after the turn back, no bump or bang, no smell and no radiant heat effects, reduced the chance of panic.
Perhaps a light by each door that indicates if the nearby engine is running would be an aide to cabin crew to asses saftey of slide deployment? Especially if passengers are becoming rowdy and the sound of engines may be drowned out?
Mickjoebill