• How can I access the filtered SPAM mailbox?

You should enable Spam Box in your cPanel.

• Webmail spam box had many items in the first couple of days, but now no more, why?

• Some of filtered items were not spam. What should I do in this case?

In this case you can use Whitelists. Or you can use cPanel 'User Level Filtering' and 'Account Level Filtering' features.

• Can the Spam filtered mail be downloaded to Outlook for a quick inspection before deletion?

You can use IMAP protocol in your Outlook to be able to manage the spam folder.

• How filtering can be used to allow email to pass through rather than just block it?

SpamAssassin does not bounce a message itself. It only adds header to the message. So you can add your rule to Account filtering to move marked messages with some specific spam status/score to one of your directories.

As for the whitelists, you can set them in your cPanel for SpamAssasain, so messages from those recipients won't be marked as SPAM. Also you can use text file in your homedir and put there your custom rules where some regexp will add some score, etc.