Electronic – Cause for failure after short circuit Step-Down Converter Input

Very nice circuit!

To answer your questions:

1. This circuit is not able to replace a fuse, because it does not protect in case of a short circuit, it only gives a warning. Your diode D2 still needs to handle a forward voltage of 12V in case of a short circuit and will break as a result. As a solution to this, you could think of a circuit that turns of your power supply to the load, whenever the load is short circuit. Or maybe even safer. A circuit with memory, that turns off the power supply when the load is short circuit and until a button is pushed stays off.
2. To adapt this circuit for a negative power supply, you could connecto the 0V to the location of your current 12V and the -12V connection to your current GND connection. You might however want to have the diode not in the GND connection, and move it in the path of the -12V supply. In that case your diode D2 would forward from D5 and JP1 to your -12V supply, and you could leave out the inverter made with Q2 and R1.
3. What kind of transients do you mean here? Short over voltage of your 12V power supply?

The input and output caps are very important .The buck convertor like many others draws current in lumps ,basicly when the switching transistor inside your chip is off no significant current is drawn and current is drawn when the switch is on .The current waveform is at the switching frequency and is in a trapizoidal shape rich in harmonics and probably will have a spike on the leading edge ! SO the input cap is selected for low ESR at the switching frequency to encourage the HF crud to flow through the cap and NOT radiate along the input cable .This is why you often see in inductor in the input power wire .Most importantly the cap must be able to handle the ripple current for your product to have a long life .Generaly its easy to substitute a ceremic SMD cap here.On the output the cap must be low enough impedence to keep ripple to an acceptable level for your load AND it must handle ripple current ,not as much as the input on your orthodox chip,AND its part of the control loop in fact its ESR effects the bode plot .If you change the output cap you must check for instability.Have you considered a higher frequency approach that has no chip that reduces switching losses that will run on ceremic caps all round ? Maybe the LF chip isnt so small due to big caps and coil .