You can probably find this on consumer-grade products too (e.g. smartphones). But the issue is finding how to 'access' these settings.

Yes absolutely.
Some astronomy sensors do support it internally: the Sony IMX571, IMX533, and IMX294 (among others) do support binning. But these models are mainly for astrophotography and industrial-grade products.

If a pixel is saturated, then you can't recover the data. That's a big issue in bright areas (e.g stars, Andromeda/Orion nebula cores, ...), and that's why people stack pictures.

It's also the reason why temperature made such a difference (and why people use cooled cameras for astrophotography): it doubles every 3 degrees C.

But what really limits sub-exposure time is astrophotography is the histogram (+ mount/atmosphere stability + satellite/plane trails + sky glow).

Lower shutter speed contains more 'dark current' noise, but that's just because dark current is proportional to exposure time, but so is the main signal: The signal-to-noise ratio remains the same.
Taking multiple exposures does not help with it (other than making dark flat more convenient).

Oh shit. I just realised no - it really is. I thought I was joking. This is going to require a long and complex backstory. Sorry absolutely not sorry it'll be funny maybe but also you'll learn something.