Interesing set up, will be a great tank I'm sure! My (very inexperienced) thoughts:
Option 1)
Tell me, is it a glass tank? the reason I was asking was becuase it may be nice to open that 1" bulkhead in the overflow box up to a 1.5", and drill an additional 1.5" or 2" bulkhead in the back of the tank for the CL intake. If you go this route, you could balance two 1.5" drains in the overflow box (there are ways of doing this fairly easily and can give you a link to a thread or two as a guide). Then you could run the third bulkhead as your return and split it into two 1" returns coming over the top of the overflow box with loc-line nozzles to direct the flow (you could even split these again into 4 loc-line nozzles). I would consider using the Iwaki or similar pump for this, and use the marlin for your closed loop, reason being that you really want your flow to come from the closed loop, and should maximize your sump/skimmer's filtration efficeincy by slowing down the turnover rate to about 4-7X per hour or so through the sump. Now, the closed loop would be a cinch (as long as you have good access behind your tank--do you have a fish roon back there?) Just plumb the newly drilled bulkhead in the back of the tank to the marlin, and run the return to an OM 4-way device, and plumb that to the 4 existing bulkheads.
Option 2: No tank drilling
If you are totally averse to drilling the extra hole in the back and widening the one in the overflow compartment as sugested above, you may want to run the intake of the closed loop over the back of the tank (given you've got room back there). The only obvious downside here (to me) would be that you would have to prime the pump, but that should be easy enough to do by tapping the intake plumbing near the top with a 1/4" valve allowing you to suck the air out of the line to prime the pump. the rest of the closed loop I'd do the same way as outlined above. As for the sump intakes and return, i'd do the same thing as suggested above with the two drains and one return, but instead of widening the hole for a larger bulkehad you could just leave it be and run a single 1" return split at the top into two 1" loc lines (or 3/4'). this will definitely limit the amount of flow you'd get through the sump somewhat, since head pressure and resistance increase a lot when going from 1.5" to 1" plumbing, but that may not necessarily be a bad thing as long as you have the right return pump. as stated above, you don't need a ton of turnover, just a ton of flow. I would definitely run an iwaki 40 or 55 (or something similar) for this application--something pressure rated with 1" inlet and outlet. if you're going to tee off to your skimmer, you may want to think about upgrading to the 55. BTW, what skimmer are you using? what are your plans for lighting and livestock? sounds like a great project! you should check out my thread, "jessezm's 200g pentagon project."
these are just my initial thoughts, and i'm sure others will chime in and have a lot of different ideas, so keep an open mind! good luck!
