you certainly can put a one way valve, but if your stuff is set up correctly, you shouldnt need it. I dont have one, my DT is 75 gallons, and my sump/fuge is all in a 10gal tank, and my stuff dosent overflow when the power goes out.
Dont let a piece of hardware take the place of a properly set up system. That way when your hardware fails, your still ok.
personally, i think i would keep the tripple baffels where they are, before the fuge.. but thats just me, and my thought process.. others may have differing opinioins. the one thing that i'm wondering about is the water flow in the fuge area itself, with that particular layout, it would seem to me that most of the water would just go over the baffels and out to the pump, only a very small amount of the water will actually go down into the fuge area, i would suggest some kind of a stir bafel or something of the like. Even if its just a 1 inch tall baffel thats 3/4" in the water over the middle of the fuge area, that would cause the water to get stired up in the fuge a little bit...
again, just my thinking on it...
oh and one other thing, think about the volume, and width of the return pump chamber, the narrower that is, the more often you will have to top off your water.
oh, and i'm of the opinion that any time you increase the volume of the system, your are helping it be more stable, even if your only gonna use a 10gal tank, and thats only 1/2 - 3/4 full.. its still more stability, plus, it gives you a place to hide heaters, and like in my case, i hung my HOB skimmer on the side of my sump, and that gets it off the DT, which just cleans up the look of everything.