Update - I could not afford to keep my monitor disabled, so I purchased a new Plugable multi-display adapter. I plugged it in with the system shut down and 2.4 installed, and the monitor came up perfectly, runs great.
It appears, as unlikely as that seems to me, that the 2.5 beta1 driver may have actually disabled or damaged the Diamond adapter. Maybe it corrupted the firmware? This is out of my pay scale.
I will contact DisplayLink directly on this as well.
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