amavis + SA not marking messages as spam

You need to provide more detailed logging output to solve that: Stop amavisd and restart it in debug mode (amavisd debug, on Debian/Ubuntu it's probably amavisd-new debug, but since you mention you configured amavisd with amavisd.conf and not /etc/amavis/conf.d/50-user, I guess you are not on Debian/Ubuntu). While you do that, you will probably want to disable the Postfix SMTP port 25 listeners on your mail servers external interfaces (you can always submit mail locally) or else you will probably get a little bit too much logging, so locate those in /etc/postfix/master.cf and uncomment the appropriate entries (don't forget postfix reload afterwards).

Also, to avoid cluttering your screen, remove the $sa_debug setting from your amavisd.conf temporarily - but reenable it if you can't figure out the cause of your problem (and also try to check why autolearning fails).

Two more comments on your configuration, which are not really related to your problem:

1. Set $sa_kill_level_deflt to $sa_tag2_level_deflt - your final spam destiny is D_PASS anyways.
2. Set $sa_tag_level_deflt to undef.

A comment directed at Dave: amavisd-new doesn't use spamd by default, so restarting spamd is rather pointless. Furthermore, there is no way you can modify the way amavisd-new treats a message of a given content category (here: CC_SPAM) from within Spamassassin's configuration.