Regarding the vent business, I think it's fallacy to consider the airtightness of a starship a Boolean issue. I mean, physically speaking, it isn't and can't be: apply a thousand atmospheres on either side and something is going to give. But the ship could also have plenty of vacuum-air interfaces where a much smaller pressure difference makes a difference.
We know the cloud can push through shields, and Spock thinks this comes naturally for an entity that manipulates gravity; this meshes well with the later (largely backstage) ideas of shields being based on gravity manipulation, essentially an englobing type of tractor beam. But we don't learn the creature could push through solid matter. The closest it gets is invading the jar of blood in the end - but our heroes would have prepared the jar so that the creature would consider it a snack, so we can't rule out them punching macroscopic holes in the lid. More probably, though, they'd use a membrane they think the creature can push through.
So, why would there be a suitably permeable membrane between the impulse engine and the ventilation ducts?
Why not? There were people attending to the engine. If we assume the dangerous innards of the engine are safely behind airlock and key, we invoke airlocks - and those are definitely in need of air ducts with selective membranes. The inspection hatch Scotty speaks of could be the engine-side hatch of an airlock, and thus quite plausibly a weak spot for the gas cloud to exploit.
The ventilation system would probably be hard pressed to create the sort of counterpressure that could defeat the cloud, assuming the system features membranes that are permeable to the cloud but don't otherwise cause leakage. But odds are the cloud isn't defeated by mere backdraft - it withdraws because it is surprised and confused by the taste of Spock (not just his blood but arguably also his mind), or because it has completed its attack as usual and retreats as usual (we saw this behavior time and again down on the planet). The fans activating is just a minor boost for the entity in completing this action...
Timo Saloniemi
We know the cloud can push through shields, and Spock thinks this comes naturally for an entity that manipulates gravity; this meshes well with the later (largely backstage) ideas of shields being based on gravity manipulation, essentially an englobing type of tractor beam. But we don't learn the creature could push through solid matter. The closest it gets is invading the jar of blood in the end - but our heroes would have prepared the jar so that the creature would consider it a snack, so we can't rule out them punching macroscopic holes in the lid. More probably, though, they'd use a membrane they think the creature can push through.
So, why would there be a suitably permeable membrane between the impulse engine and the ventilation ducts?
Why not? There were people attending to the engine. If we assume the dangerous innards of the engine are safely behind airlock and key, we invoke airlocks - and those are definitely in need of air ducts with selective membranes. The inspection hatch Scotty speaks of could be the engine-side hatch of an airlock, and thus quite plausibly a weak spot for the gas cloud to exploit.
The ventilation system would probably be hard pressed to create the sort of counterpressure that could defeat the cloud, assuming the system features membranes that are permeable to the cloud but don't otherwise cause leakage. But odds are the cloud isn't defeated by mere backdraft - it withdraws because it is surprised and confused by the taste of Spock (not just his blood but arguably also his mind), or because it has completed its attack as usual and retreats as usual (we saw this behavior time and again down on the planet). The fans activating is just a minor boost for the entity in completing this action...
Timo Saloniemi