It's Tom Straw - I saw an article/interview about that a couple of years ago, though I forget where and wouldn't have the link any more.
But I'm certain Marlowe is participating too, at least contributing to the outline and doing revisions on Straw's work. As I said, the character voices are so perfectly captured that it most likely comes from the same hand that does the final draft of all the show's scripts. And the acknowledgments do thank Marlowe's family, something Straw alone would have no reason to do. I imagine something similar to how Shatner collaborates with his various co-authors -- they do the bulk of the day-to-day writing, but he shapes the plot, handles the main character's dialogue, and has the final revision and approval for everything.
I gather that the authors won't publicly acknowledge their authorship until after the TV series ends, so as to maintain the illusion.