1.
You need to determine the coil's Henry value, based on the given switching frequency and load current.
The LR time constant is useful here. Start at switch-On time. Current ramps up from zero to, say, 2x your load current. Then you switch Off. Current ramps down to zero.
The cycle period is 1/18000 sec.
Your duty cycle prospectively is 5V / 12V, or 42 percent.
Assume a switch-On resistance of a few tenths of an ohm. What Henry value will ramp up to 4A during switch-On?
During switch-Off your load resistance is involved. It calculates to 2.5 ohms (5V divided by 2A). What Henry value will drop from 4A to zero A, through a resistance of 2.5 ohms, during switch-Off?
The above approach can only be approximate, because the smoothing capacitor adds its own dynamics to the mix.