Fixed all errors.
Works well once installed as a privileged app:

C:\> adb reboot bootloader

C:\> fastboot boot android\unsecure-hammerhead-boot.img
downloading 'boot.img'...
OKAY [  0.406s]
booting...
OKAY [  0.109s]
finished. total time: 0.516s

C:\> adb shell mount -o rw,remount /system

C:\> adb push bin\BestUsbTetherWidget.apk /system/priv-app
Introduced "auto-start" checkbox, and it appears to work!
Merge remote-tracking branch 'origin/master'