I get notifications for spam all the time, and it shows up in my NC. I have two personal accounts, two work accounts and an account for a small side business. In the mail app, under the main mailboxes list, I added all my archived email folders, sent email folders, junk (spam) email folders and trash email folders. This provided me with quick access to any folder I wanted to review. What I soon discovered was that any folder that is visible under the mailboxes list will notify you in NC for each email that is received. So if you want to receive notifications for spam email, just make it visible in your mail app under all your inbox list. As for delayed notifications, I'd follow the instructions above and set the delay between notifications to the maximum amount allowed.
Nerd