Mac OS update has an invalid signature

I am trying to update my Mac OS Mojave which is currently running 10.14.3. Now the latest update is 10.14.6. I tried updating via system preferences but it gave me an error "This volume doesn't meet the requirements". I tried downloading and installing combo update from here: https://support.apple.com/kb/DL2010?locale=en_US It gives me error "The package has an invalid signature" on installation while installing after restart. I tried downloading and installing 10.14.4, but it gives the same error "The package has an invalid signature" during installation.

I have been stuck here for days. I really want to update my mac. Please help.

-- diskutil list output --

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *121.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         121.1 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +121.1 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            97.9 GB    disk1s1
   2:                APFS Volume Preboot                 42.0 MB    disk1s2
   3:                APFS Volume Recovery                517.1 MB   disk1s3
   4:                APFS Volume VM                      1.1 GB     disk1s4

My security and privacy pane is also not loading. Can the update installation have something to do with it?

It sounds like it could be a corrupt plist file or something else entirely. You didn't happen to remove any certificates from Keychain did you (accidentally)? Apple uses those for verifying code signatures.

One simple idea to try is to create a new admin account on your computer. Login and see if you're able to update the OS from that account. You can delete that account after the system if updated, or at least, you've now isolated the problem to a single account.

If that fails, I might try just re-installing macOS from the recovery partition. This will overwrite the system files but shouldn't affect anything else.