I would go ahead and book the tickets. You can always redeposit the miles for a $50 fee. Since award availability is always hit or miss, you should definitely book the tickets if seats are open.
Did you try to book the tickets on the web? If so, it may not show all of your options. Calling Northwest might help. When I do this, I am always given more flights than the web is showing. (of course you don't get the discount for booking online)

Good luck!