I will especially use the fact that it is meaningless. When something has no meaning, you can do whatever you want. A derivation is not something you look forward to on Friday. Hopefully time will run out so I won't have to do it. Mandl and Shaw has a rather mysterious derivation.
Light travels with the velocity of light.
That's a nice sentence. The best thing in physics when you're in trouble is to sit down and calculate. It's always refreshing and healthy. When you have a whole mess, it's safe to assume that it doesn't vanish. The role of the propagator is to propagate.