Solved! This evening I checked the power connector to each camera's active USB 3 extension cable to be sure the 5VDC "brick" was producing voltage. I couldn't even see the metal contact inside he coaxial connector for the guide camera's dongle, let alone get a meter probe to touch it!

I brought the power brick indoors, used an X-Acto blade to trim off excess plastic from the center insulator, then used a couple of dental tools (compliments of my hygienist) to reach the metal contact and bend it inward.

When it got dark, I started my M5 plan, and the guide camera worked perfectly. Sadly, just as I was ready to start a "for-real" plan, the sky clouded-over, and ACP couldn't find even one guide star. So I quit.

But, nice to know what the problem was, and that it's fixed.

Side note: I ordered an Icron Raven 3104 4-port USB3 extender (up to 100m over CAT6/7 cable) to replace the two active USB 3 extender cables. I have an Icron Ranger 2304 USB 2 extender, and it works great. I'm told on the AP-GTO list that the USB 3 device works well, too. I decided to go for the gold -- I'm tired of fooling around with halfway solutions.

--- Mike