Jens so I finally have a resolution to my issue, as it turns out it was the bent pins (pictured in my second post) causing the problem. I assume they were not making contact with the Amiga.
I wired up the new RT-65B power supply (tuned the 5v line under load) and again had the same "black screen" issue as before. Disappointed with the result I decided to try to bend the pins back into shape. I used a pair of very fine tweezers that I use for SMD soldering to bend them as close as possible to the other pins. I used a pair of 3.5x magnifying glasses to make sure that I wasn't shorting anything out.
Sure enough after re-seating, the card worked immediately. I'm very glad to finally have the issue resolved, and have my aca500plus working!