Voltage monitor circuit (reverse polarity protection with high voltage cut-off)
Your circuit has issues and I would not try to "patch" these as there are simpler, more reliable solutions!

I would use a poly fuse which is a self-recovering fuse. After it blows it will recover (become "good" again) after some time by itself. Of course you could also just use an ordinary glass fuse but make sure that the user can replace it without opening the case.
There are fuse holders for this purpose.

So I rely on the fuse to cut the power when the DC supplied is not OK. That is safe and reliable. Here's my circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

The diode D2 conducts when DCIN &lt; 0.6 V so current flows and the fuse will blow.

During an overvoltage the SCR is triggered through D1 which shorts the supply, causing a high current to flow through the fuse which will then blow.

Also note that my circuit is much simpler than what you propose :-)