Disconnect the computers from the physical network. If the domain user credentials have been cached on the computers this will allow the user to log in with those cached credentials. You can then take whatever steps are needed to reconfigure the computers.
Additionally, contact Dell support again and ask them if there is a way to disable or override the BIOS ...
The problem is the filter. The string (!displayName=*,*) indicates that only user who currently don't have a , in their displayname should be changed. You want the inverse, so you need to remove the !.
Get-ADUser -LDAPFilter "(&(objectCategory=person)(displayName=*,*))" |
Set-ADUser $_ -DisplayName "$($_.givenName) $(...
I managed to make GSSAPI work using the following tutorial:
What I did (I'm on Debian)
Join the domain
apt-get install --no-install-recommends winbind smbclient krb5-config krb5-user libldap-common
kdc_timesync = 1
ccache_type = 4