question on schwarzschild effect

Hello there

I'm new to this forum and already have a question.
It's about the Schwarzschild effect on long exposure times and I'm not sure whether I got this right. At very weak light, the

photon flux is very low and it can take a very long time until a crystal (within the emulsion) was hit by enough photons to

build up a stable and developable silver core. But it can happen during this time that an already reduced silver atom is reoxidised and thus loosing the effect of the first photon. Is that right so far?

But photons don't come ordered but they are straying randomly around a mean. Thus, it is still possible that a crystal is hit often enough in a sufficently short time to build up a stable silver core. Though it is becoming nonlinearily unlikely (but not impossible) with decreasing photon flux, which might explain the increased exposure time at long exposures (the schwarzschild effect). Is that still right?

But is there an ultimate threshold of lowest possible (but photographable) photon flux? Or is it possible to extend exposure time to hours, days, weeks if neccessary to get a stable latent image? But: there are always photons zipping around, even in practical darkness due to ultra weak photon emission for example. And in case there is no lower limit of the photon flux (and no upper limit of feasible exposure time), wouldn't this mean that over time even a packed film would be exposed? Or is this actually happening and thus a reason for limiting shelf life of undeveloped films?