These old fire curtains are usually associated with a haystack lantern light above the stage that vents the space in case of fire.
[For general info: The vent opens in the event of fire, on a fusible link or manually for example, and raises the neutral pressure plane above the stage to a position somewhere above the gaps, that kurnal referred to above, between the curtain and the proscenium arch. The neutral pressure plane rises because there is very little inlet air at low level but the buoyancy of the smoke drives a lot of gases out of the vent at the top. This means that there is a drop in pressure in the space immediately behind the fire curtain and so the gaps will allow air to pass from the auditorium to the stage but will not allow smoke to escape from the stage to the auditorium.]
If there is no haystack lantern light above the stage in this case or if there is one but it has been decommissioned then the neutral pressure plane will stay low and smoke will escape round the curtain to the auditorium. So the curtain won't do a very good job.
If there is a haystack lantern light then the opening of the vent(s) might adversely affect the operation of the heat detectors, though, as you say, they're bound to go off by the time the fire reaches a size where it might start to buckle the fire curtain.
I would initially be inclined to recommend keeping it manual only but I do understand that if this is an old theatre then there might be some heritage value in the auditorium that deserves protection when the theatre is empty.
I would check to see if the ventilation is there. If not then integrity could be provided either by ensuring that the fire curtain shuts tightly onto fire resisting stops all around the arch with cold smoke and intumescent seals (which would be very unusual) or by providing an additional, modern fire curtain in front of the old one. I imagine the old one will have some historical value and will not be able to be removed.
As for the drencher, fusible links/glass bulbs would offer the most resilient and simple solution if the owners (or their insurers) are determined to make the system automatic, but in this day and age why keep things simple when you can over-complicate them? Also, I do not think that the drenchers are normally there to put the fire out. If that was their purpose why would they be directed at the curtain? Maybe in this case they're not...
