So my questions are:
A – Do you recommend 1, 2 or another design?
B - What transistor would I need (reference?)
MOSFET, check sparkfun, digikey, datasheetcatalog or google
Alternative - BJT, TIP120 / TIP140 cheaper, same time you 'd need a heatthink
C – Should I use an optocoupler or something to protect the Arduino? (reference?)
I'd recommend, inductive load creates a EMI nightmare .
4N25 / ../ 38, MCT6 practically anything if PWM freq. < 1kHz, 6N136 / 137 if above
D – Somewhat unrelated: I have the feeling that my design is wasting energy in the resistance.
How could I maintain the 2 amps without having the resistance getting hot? (If I had a 2V power source, I wouldn’t need the resistance at all) how can I get a 2V/2Amps source from my 12V/2amps without a resistance getting hot?
There would not be any resistor in B.