Yes, it will be quicker.
Yes, it will be quicker.
Replacing the motherboard and CPU doesn't necessarily mean you need a new product key.
Yes, you can generally use your old hard drive in a new upgrade, but it depends on compatibility and condition. Check if the new system supports the drive type and format.
With an OEM key, swapping your motherboard might render your license unusable. If you have the Windows installation on the drive, it's usually best to reinstall Windows after making changes.