Yes, you've been asking lots of them.....
In another thread you mentioned doing a 8.03 to 9.0 upgrade. Most of this hasn't changed any in SBG 9, other than how LDAP works and that mail quarantine is now on the CC filesystem instead of the MySql database.
1. Scanners only have basic traffic logs, service logs,(see admin, logs for a list - except for Directory Data Service which is scanner only). They also have any queued messages on them.
2. Mostly flat files. there are command line utilites to view the logs (see malquery, monitor commands)
3. The SQL database is only on the control center.
4. All the logs do, and anything that ends up in a spam or virus quarantine; or content incident folders.
5/6. Not sure, around every 5 minutes. E-mail object get sent back to the CC via SMTP protocol over the control center ports (see the install or admin guide for what ports you need open.
From your questions, I hope you are not yet in production and just getting to know the product.