Q1: what are fly-back diodes, what's their purpose and what values should I use(Are 1N4004 or 1N4001 useful?)
When you have current running through a inductor (coil of wire like a motor winding or solenoid) it forma a magnetic field: turns the coil into an electromagnet. When you disconnect the flow of current the magnetic field collapses rapidly. As it collapses the change in magnetic field through the coil induces an electrical current opposite the original current. This can cause a large reverse-current spike. The flyback diodes short out this reverse current spike to prevent it from damaging other semiconductors in the circuit.