I know there are so many possibilities. For eg, what if the fire in the roof space is located quite a distance away from the smoke detector(s) under the ceiling. The further the smoke travels away from the fire the cooler it could get, to a point where it may not be buoyant. Could the smoke start to filter down through any gaps in the ceiling and activate the smoke alarm? Would that be a more likely scenario then pressure build up squeezing the smoke down and out (considering you may only have a fire whose HRR is being regulated/restricted by the available oxygen).

On the question of -vents in the roof and the predominate flow through charred slots generally being upwards-. What is flowing upwards through the slots? Is it the air below the ceiling? Wouldn’t the cooler, denser air below the ceiling be stably stratified and therefore not flow against gravity?