1. YES
2. This is doable i think with big main unit + proper cooling, maybe a active fan, and the main unit as a remote one so no direct sunshine on it,,,,,,, even if cars do get very hot indeed.
3. I have long asked for SSD or NVME storage instead of silly SD cards, this do require a smart offload routine to a thumb drive or via fast WIFI. this ,might also better allow to do encrypted drive, which are requested now and then.
4. IR light dont have to be separate, but the few true night vision dashcams are few and far in between,,,,, as in have a active IR cut filter.
5. 4/5 G modem would not have to be remote, but if it is in a remote main unit maybe deep in the car, it and or wifi might need a outside antenna as what are build into cameras today are pretty weak antennas not meant for range.
6. Station mode i think will soon be the norm, if it only support one or more guest users like a CCTV system can be set up to have i dont know.
7. This depend on proper fast wifi i think, and of course what is in between and the APP to properly take advantage of this.
8. If you have a main unit remote from the cameras, what camera module you connect could be individual, if you have more main units they should be able to sync up, universal side cameras though are hard to design as cars are very different here.
9. this is standard now i think.
If you make a remote main unit it can be pretty large, and so house a proper heat pipe cooler, and as it it hidden a fan could also be used cuz after all in a hot cabin transfer of heat from cooler to air are slower than in a cold car, but exchanging the air over the cooler fins should help VS a completely passive system.
I would galdly take a remote unit that is 5 X 10 X 20 CM in size