It depends on how you do the installation. Some OS allows you to modify the partition where the OS will be installed into. Some will just wipe up the whole disk.
This is not true. OEM serial key is usually tied to the motherboard. I once had to replace my HDD, but granted i downloaded the Windows recovery file from the laptop manufacturer's website. That way it will recreate the recovery partition for you.
And it's also not publicly available for everyone.
You forgot the original series from the 90s, Gatekeepers and Gatekeepers 21.
Isn't this exactly what Netflix is trying to do with their paid subscribers?
On personal PC maybe not, but when you're using a Windows PC for work that is enrolled and controlled by your IT, the update can be enforced and there's no way to postpone it from your side.