I just drew a yellow line,
Aslong as your water level is above this yellow line then water level will be exactly same in left chamber and middle chamber.
Once water level goes below the yellow line then left and centre chamber are independant from each other till something changes ( if water level drops below yellow line,im taking it the system has no ato or the ato has failed or ato water chamber is empty etc,and if water level goes below the yellow line then left chamber where you plan on having your return pump will keep pumping water back to main tank and at risk of pump running dry so highly likely pump will fail and return pump in a sump like a heart in a human body pumping blood around body as sump holding alot of filtration like skimmer/carbon and more than likely heaters so very important keeps running.
If power heads for flow in dt and live rock then dt filtration will still function as the live rock is our greatest filtration and need flow,( always good to point power heads at surface for some surface agitation so promotes gas exchange,c02 out and oxygen in depending on c02 levels in room tank is in,but without a heater the temperature will ovbiously drop so depending what your ambient room temperature is will depend how much temp will drop and become big problem or not,your skimmer ovbiously takes organics out system before fully broken down so is very beneficial but not 100% essential,but also aerates your water adding more dissolved oxygen to your sysyem and therefore highers ph so beneficial,same as carbon unless trying remove something you dont want etc,so having 2 smaller return pumps connected and running is better than just 1 and often not much price difference and if one pump fails you still got one pump running keeping all equipment in sump in operation and have extra water volume so system stays more stable and say if a heater/s failed a higher water volume generally means water temp takes longer to drop.
Sorry for long winded reply,i got little carried away so had to stop before turns into an article lol ^_^