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.

Everything was working perfectly for both of us (others as well I'm sure) Only thing that changed is we installed the beta. At home where I installed the beta, the adapter is toast. In my office, all 3 of my monitors fire up no problem running 2.4. And I should add it's playing nice with El Capitan for now as well.

While I anticipate some bugs in a beta, I don't anticipate losing my device functionality no matter what I try. I'm 100% sure the software disabled the adapter either temporarily or permanently.

Just plugged in my new adapter at home and voila, all is well again (still running 2.4). Thank you Carlo and Wim!