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.
Yeah If you use the same frequency as an existing station.