First of all, I'm new to Drupal and the work I have to do is some kind of too hard.
My old colleague (webmaster) had a server with a multisite Drupal 6 installation.
Sites and their dirs were (e.g.)

Sites              Site directory
b.a.mycompany.com  /drupal_install_dir/sites/b.a.mycompany.com
c.a.mycompany.com  /drupal_install_dir/sites/c.a.mycompany.com
d.a.mycompany.com  /drupal_install_dir/sites/d.a.mycompany.com

Unluckily my colleague moved and server hdd aren't in my hands: all I have is a backup of /drupal_install_dir and three sql dumps (one for each site).
I had to restore three sites, but changing them as

z.mycompany.com/b
z.mycompany.com/c
z.mycompany.com/d

Beeing a sysadmin, I

1. Extracted tar.gz backup file under wwwroot (let's call full path to extracted directory /new_install_dir)
2. Restored three databases
3. Created mysql users and give them correct GRANTS on databases

Then (trying to restore at least first site) I changed /new_install_dir/sites/settings.php putting correct database connection data and new basepath.
But there is no way I can see my new site, simply it doesn't work.

Watching /var/log/apache2/error.log I saw Drupal searching for main drupal database; so I created that db too setting user and grants, but dump file is empty. Well, now I can run something like install.php or update.php, but my site is not shown.

Is there something I can do?
Do I have to walk another way? Consider I searched the web, but I'm not able to find a guide that can help me for my problem.

Ah, I forgot: before producing the backup, my colleague set site in maintenance mode.
When I try to run z.mycompany.com/?q=user (trying to login) nothing happens.

I'm really stuck...