I'm the author of the tutorial.
a) you must set the name of your workgroup. If your domain name is domain.com, your workgroup name must be DOMAIN. If your domain name is mycompany.lan it is MYCOMPANY
b) the UID and GID of your users from your domain are very high numbers, because you left commented out the line ldap_id_mapping in sssd.conf. If you want to see the AD users in the user interface of OMV, you ned to set MAX_UID and MAx_GID to a value greater than the values ID of your AD users and groups. This documentation may help you: https://linux.die.net/man/5/sssd-ad
c) nice to know. I may update the tutorial to warn about this important change. (Did I write a version for OMV 4.x ? I don't even remember).