1
You can install Windows 10 or 11 anytime you need to on an empty drive by using the Media Creation Tool to create a bootable Windows USB on an empty 8GB USB flash drive, then boot your PC from that.
Windows 10:
https://www.microsoft.com/software-download/windows10
Windows 11:
https://www.microsoft.com/software-download/windows11
2
Activation after reinstalling Windows depends on what type of Office licence you have.
If Windows was pre-installed on the PC, then there is an OEM licence stored on the motherboard, and Windows would activate automatically after you reinstall Windows.
Otherwise, if you purchased a full retail Windows licence from a 3rd party retailer, you would need to use the troubleshoot method on the Windows activation window in the settings app to try to reactivate Windows.