yes - a status of delivered means that the receiving mailserver sent a 2xx code (meaning it accepts the mail for delivery) when PMG tried to send the mail
no for both questions - in general a mail-server only knows if the receiving mail-server accepted the message - not if it's further delivery succeeded.
there are some RFCs which specify ways how to automatically notify a sender that a message was received by the recipient, but those are not provided on the public internet in most cases (to not disclose too much information). Some mechanisms exist where the recipient can actively acknowledge receiving a message - request return receipt - but it depends on the recipient to actively say they want to sent that information.
The tracking center takes its information from /var/log/syslog and its rotated variants - those get written by rsyslog, and are regular Linux syslog files - you can use any technique that store syslogs in an audit-proof/revision-safe way .