"adb devices" successfully shows device, but "adb connect usb" inevitably fails, any suggestions?
Here is the documentation on usage of the adb utility.

Basically, adb connect is only needed if you are trying to connect to a device over TCP IP, i.e. wireless network.  You don't need to issue any "connect" commands for a device connected via USB if it is already recognized in adb devices prompt.  Just issue any other command, e.g. adb shell ls /sdcard/ and it should connect.