I was able to demonstrate that an OpenMV cam disconnected from my custom adapter PCB was able to run standalone, even using UART1. I then proceeded to swap the header orientation on this OpenMV cam (so I could connect to my custom PCB), and somehow managed to kill that OpenMV cam in the process. I attempted to reflash the firmware on that cam, but it is not recognized by the IDE - not sure what happened.
Upon plugging back in the OpenMV cam which was already connected to my custom adapter PCB, the IDE started re-flashing the firmware. Oops. But, once the re-flash was complete, it could run my basic LED flash/hello world program.
So, reflashing the OpenMV cam seems to have resolved the standalone operation issue. But the firmware version is still 3.0.0 (as it was before).
I still need to confirm that I can run my actual program standalone - but this is progress nonetheless.
Any ideas what may have caused this issue?