If I consider the reasons why Snape might fabricate Draco's death, I can only arrive at a few. Tosha may have more insight.

If Draco is indeed alive, there is much, I'm sure, he could be convinced to tell us.

But we must work with what is available to us right here, right now. I've secured our Lord's approval to move forward. Hydra will be in your hands, soon. My greatest concern is that her fondness for Harry will cloud her ability to do what must be done. She's showed mettle of late, but still has much to learn.