I got it, but what I want to know is if anyone has had good luck upgrading from the standby list?

According to the rules spelled out below from the SWU certificate: You wait list and then 3 hours prior to departure you asked to be placed on the standby list.

Second question is does a Diamond ever trump a Non-Rev? What are Delta's priority rules?

Passengers on the Airport Standby List will be accommodated according to Delta's priority rules, which Delta reserves the right to change at any time.
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Reservation Waitlist: If the upgrade is not available at the time of request, a Medallion member and/or travel companion holding confirmed Coach Class reservations on a flight may request to be placed on the upgrade waitlist. Passengers on waitlist will be accommodated according to Delta's priority rules, which Delta reserves the right to change at any time. Waitlist is not available on KLM operated flights.

Airport Standby: If an upgrade for the Medallion member and companion (if applicable) is not confirmed from the reservation waitlist at least three hours prior to departure, then the Medallion member and companion may request to be placed on the Airport Standby List, provided that request is made no less than 60 minutes prior to flight departure. Passengers on the Airport Standby List will be accommodated according to Delta's priority rules, which Delta reserves the right to change at any time. Airport standby on KLM operated flights is handled at the discretion of KLM and is not guaranteed.