User has switch set as 12V but connects 24V - Could I put in a fuse to make it "break" on purpose?
If you just want to ensure that the load is not going to see 24V, instead of sensing the source, why don't you add an over-voltage protection at the load side. Maybe a crowbar or a back-to-back switch should work fine. See circuits below
User sets switch to 12V and connects 12V - What happens to the output side of the 24V regulator, it get's 12V and loads up the inductor ? Do I need to stop the 12V from going there and if how?
You could add diodes at the output of the buck-converter and at the 12V line, thus decoupling them. See circuit below.
How it works?
If the output voltage is high enough to turn on the zener diode, some current will start to flow through it thus trigerring the SCR, thus short-circuiting the input and blowing the fuse.
If the zener's tolerace does not suffice your needs, you could replace it with a reference voltage e.f. TL431. The sensing logic would have to be adapted accordingly.
simulate this circuit – Schematic created using CircuitLab
Here is another option with back to back FETs.
How it works?
Initially both FETs M1 and M2 are pulled to ground through Q1, therefore they conducting. If the output after the diode D2 is big enough to turn on the diode D4, the output MOSFETs will be turned off.
simulate this circuit