Only because this response is easy...
Using the Emperor's DNA requires putting a needle in the Emperor and using what comes out.
No, because Corax is Corax. Not the Emperor. That's like saying that your DNA is the same as your Dad's. That's why they turned out as Super-Raven Guard instead of Primarchs.And surely, since Corax is the Emperor's son, he has the Emperor's DNA in his own blood?
Why didn't Dad just give Corax his DNA? Well, because what happened later is exactly what the Emperor was afraid would happen (and he warned Corax too).