Wallwarts are cheap. If I were making such a circuit, I would use a separate
power supply for the sensors, so the Arduino would always stay active in
case for remote shorts.
I would place a small value R in series in the high side of the power to the
remotes, to sense current flow differentially using voltage dividers to the
Arduino A/D converter pins, and use the Arduino to control the pass
transistor, Q1. However, I don't understand how your ckt with the SCR is
supposed to work [some kind of crowbar?]. You don't really need a crowbar,
just a way to turn on/off Q1 using the Arduino.