1. When I try to access roundcube it says that the database cannot be accessed.
Did you check SQL username/password set in Roundcube config file (config/db.inc.php under roundcube directory)? Is it correct?
2. the test message at the end of the installation IS NOT delivered and there is no directories at all under vmail1. (the postmaster mailbox is not there)
Any related log in Postfix/Dovecot log files? Maildir will be created automatically when delivering mail.
3. I cannot configure thunderbird for the postmaster account... It keeps "Checking password" (probably because the iRedMail system cannot access the database at all, right?)
Then i guess its password is wrong during iRedMail installation. Does reset a password work for you?
4. Basically, nothing works.
I did a test and installed iRedMail using the traditional instalation with a local mysql database and it worked. The server is correct, DNS is correct, etc... But I REALLY need to store everything in my percona cluster! (the cluster is a production cluster and IT IS working for all other applications I have on it)
Please check related log files to see why everything is not working as expected.