Email-in Filtering or Multiple Emails per Category/Group

If you’re willing to filter on subject or contents rather than to-address, I think your first use case might be something that Discourse Automation could handle. Have a hidden “incoming mail” category and a trigger which fires the script when there is a new post there. That script would have your custom rules and based on those move the message to the right category (or tags!)

The second and third seem like they might be better handled by an email forwarding service external to Discourse.