[JacobAziza]

disclaimer: I don't have any electrical training at all. Like everything I do, its all self taught and largely trial and error.

it looks like the diode is in parallel to the relay, in which case I'm not sure what good it would do. But then again, I can't imagine why it would be necessary anyway. Where did you read that, and what sort of "bad things" did they suggest?

With the resistor in line in the parallel version I suspect it would affect the headlight less than if it was in series (though, again, I don't really know much electrical theory)

As far as the relay power, I'm not sure if it is contact (switch) or coil or both, but considering they are usually used to use a low current switch to control a higher current circuit, I would expect the rating to refer to the switch side.
It certainly won't hurt to have one rated for higher than you need in any event.

What might be slightly more challenging is finding a normally closed relay.