You need to do two things: -
- In red is the corrected MOSFET position
- In purple is protection to your gate voltage due to excessive supply voltages
It's not a purely resistive load, and the regulator it's powering also powers inductive loads itself (12V solenoid valves)
Rely on the diode clampers directly on the solenoids - don't try and protect the MOSFET at a distance - protecting at the source of the problem is all you need to do.
what form factor product can I search for that provides an easier, more modular solution for placing FETs inline on the wire, similar to a fuse assembly?
It depends in the main on the current consumption of the regulator and load. Pick a MOSFET that can handle at least twice the incoming supply voltage.
Make sure that the regulator turns off on "under-voltage" before the MOSFET gets used in its linear region because it might fail. This requires careful examination of the data sheet for the regulator to ascertain what under-voltage protection it has.
what the dangers would be after disconnecting the board mounted regulator at Vin.
I'd put an extra bleed resistor of about 10k to 100k on the terminals that connect to "Vreg 24 V" (a confusing label) on the left of the diagram.