Switch 12V loads with MCU and BDW93C darlington or IRF630N
Q1) I would guess the cause is current taking a while to pass trough the high input impedance of the voltmeter, try connecting a resistor in parallel to see if results are more stable.

Fan: It only draws 0.3A and so the BDW93C would work just fine, as the fan has 3 pins, you switch the power and ground pins and leave the 3rd pin unconnected (pin for speed sensing) 

Boost converter: this boost converter cant turn off completely, only stop boosting voltage. You can use an N-channel MOSFET controlled using a drive circuit.

The reason for needing a drive circuit is that the MOSFET barely turns on with 3.3V, one circuit you can use is this:







simulate this circuit &ndash; Schematic created using CircuitLab

This has 1 problem, setting the MCU pin high will turn off the converter and setting it low will turn on the converter. Also if the mcu is not active, the MCU pin will float and the converter will turn on.

One way to fix this is:







simulate this circuit

This circuit uses both a PNP and an NPN however, it will prevent the MOSFET from turning on if the MCU isn't driving its output.