It's not that important to have a balance of lefty/righty starters. I believe you need at least one lefty, but it's much more important to have at least 2 lefties in the bullpen. So, I don't think that will factor into the starters equation.
Detwiler is out of options, and would need to pass through waivers to leave the 40-man and go to AAA. That will not happen, so he's either on the team or gets traded. Lannan does have one option, but as a practical matter, a guy with 4+ years service time and making $5 million guaranteed is not going to the minors as an insurance plan.
As stated earlier, Wang cannot be traded until summer, but I could see him pitching out of the bullpen as a long reliever. Detwiler too, but not Lannan.
So, assuming no one gets traded and no one gets hurt in Viera, the staff could very well look like this on opening day.
Starters - Gio, Strasburg, Zimmermann, Jackson and Lannan.
Bullpen - Wang, Detwiler, Burnett, Rodriguez, Lidge, Clippard, Storen
Where's Tom Gorzellany? Well, he's probably better trade bait than Lannan, since he can start and relieve, and his contract is a lot smaller. I can easily see the Nats parting with him for a minor leaguer or two, and for more teams to be interested due to his versatility and price. Or the Nats just flat-out eat his contract.
This gives Davey his two long relievers, plus his #6 and #7 starters already on the 25-man. At AAA you stash Mathieus, Stamman, Kimball and the usual crew of AAA/AAAA guys the Nats always seem to have a surplus of.