mailbfr does an incremental backup by default. So unless you change the backup location regularly you may end up with old messages. This is by design otherwise you couldn’t go back to an old messsage a user might request. If you do not wish this behavior, use the "full" or "rotate" option.
No. If services need to be stopped/started, mailbfr will do this for you.
mailbfr uses the root crontab. You can either edit it directly or use mailbfr -i to add/remove the cron job.
Basic instructions are given in the enclosed documentation. All options are listed in the online documentation. To see it, issue "man spamtrainer".
Both. You will need to tell spamtrainer the filename and its location. For example: /volumes/mydisk/mydirectory/myfile