Will MK3S MMU2S work without the MMU2S unit?  

Re: Will MK3S MMU2S work without the MMU2S unit?

It works perfectly without. If it's not detected during boot it won't be used and the printer behaves as a simple MK3S.
You can even remove the chimney as well, disconnecting the IR sensor. Then you can open de idler door a little bit at least. Just don;t forget to toggle the sensor in the settings.

Re: Will MK3S MMU2S work without the MMU2S unit?

It's what I was thinking. I just wonder if you can still use the sensor for filament run out. It won't be able to unload the remaining filament, but it would at least pause and you can try to load more filament on top of the remaining bit. I just wonder how the firmware would handle it (now and in the future). I assume with the MMU2S unit it uses that sensor for run out and not the IR one.

Re: Will MK3S MMU2S work without the MMU2S unit?

It's what I was thinking. I just wonder if you can still use the sensor for filament run out. It won't be able to unload the remaining filament, but it would at least pause and you can try to load more filament on top of the remaining bit. I just wonder how the firmware would handle it (now and in the future). I assume with the MMU2S unit it uses that sensor for run out and not the IR one.

Could be, just be careful of the following:
The sensor trigger behavior is different between a regular MK3 and a MMU2S-MK3:

With the regular version, the sensor is triggered first and then the filament gets to the gears. So when the filament runs out you'll still have some between the gears when the sensor goes "0"

With the MMU2 chimney on there, the sensor is triggered by the lever, which is "mounted" to the idler door, meaning the sensor will go to "0" when all filament is gone from between the gears. That's a lot later and might be harder to resume from.