Hello
For the antivirus software issue:
Have you installed any third party antivirus software before?
If you have installed the third party antivirus software, the built-in windows defender feature may be disabled automatically.
Check the following registry keys for any third party antivirus software.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Security Center\Provider\Av
Refer to the following link to check the related service registry keys.
For the update error, did you get any error message when you installed the update manually?
Or the machine is stuck in updating process forever?
If the machine is stuch in update process, we could try to boot into safe mode and disable the windows moduler installer service from the services.msc control panel. Then the machine should boot normally.
If there is any third party antivirus software installed, please remove it temporarily.
For advanced troubleshooting, we could check the C:\Windows\Logs\CBS\CBS.log for more details.
If we didn't want to troubleshoot the issue, perform an in-place upgrade reapir with an ISO media or a clean reset may be more effieient. Please take a backup of the important data before we take any actions.