I rechecked the migration tool and there must be no AD provider installed on NS8 to make the migration work so yes, you would need to roll back everything and completely start over with the migration.
Do you mean including samba file shares in Nextcloud by using the external storage app? Yes, that works.
It’s also possible to mount a disk to use it in Nextcloud, see also Ns8 storage management - #2 by mrmarkuz