My mac says there is an authorised computer to my account but doesn't allow me to de-authorise it

You can't view what computers are authorized but you can "Deauthorize All" and then authorize any computers that you still have and want to used (note that iPhones and iPads aren't affected, only computers). See the instructions in the "If you need to deauthorize a computer that you don't have access to" sections of Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

If your Mac is running Catalina or later, use the Apple Music app to do the above as described in the Mac section of that article. If your MacBook is running a version earlier than Catalina, use iTunes to do the same thing.

Note that iTunes authorizations have nothing to to with other issues described in your post such as unauthorized access to your Mac. It only affects what computers can access your iTunes purchases.

The "Deauthorize All"option is in a different location than the deauthorize one computer. See the following from my previous post for the instructions for deauthorize all:

See the instructions in the "If you need to deauthorize a computer that you don't have access to" sections of Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

If your Mac is running Catalina or later, use the Apple Music app to do the above as described in the Mac section of that article. If your MacBook is running a version earlier than Catalina, use iTunes to do the same thing.

You may want to contact Apple. See: Contact - Official Apple Support