Turnover is somewhat irrelevant to pump run time. Some pools only need a 1/2 a turn per day while others require multiple turns per day so there is no one size fits all number. You just need to run the pump for as long as necessary to keep the pool clean and chlorinated. Circulation is also important for the distribution of chlorine but that can usually be done in less than 30 minutes so it is usually not a major factor in determining run time.
However, many Intex pool owners have found that the 1500 GPH pump does not deliver enough flow rate (return exit velocity) to provide adequate water movement so the skimmers can work and all parts of the pool get enough chlorinated water. So many have moved to the Intex 2500 GPH pump and found much better performance.
As for your other questions:
No, they are usually side by side so only one trench is required back to the equipment. If the return is separated from the suction side by the pool, they are brought together before going back to the equipment. There is no reason to keep them separated.I'm pretty much ignorant of the 'circulatory system' of a typical inground pool, but I'm guessing that the underground piping is set up so that suction and return points are widely separated -- is that guess accurate?
This issue seems to be return exit velocity. The flow rate is so low with the 1500 GPH pump that it doesn't seem to move the water very much. Based upon other Intex pool owner experiences, the 2500 GPH pump/filter should do the trick.Maybe the donut pool setup necessitates a higher pump flow rate (higher than seems needed for turnover) just to stir the water semi-adequately?
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