I would use 2 x 1.5" drains for redundancy, just run a pump that maxes out 1, then if one clogs/fails you dont have a problem.
I wouldn't put too much flow through, a mag 9 would be plenty. Don't rely on your return pump for too much flow, you can run into micro bubble troubles.
Depending on your skimmer type, you could match up your return pump to the amount of water volume your skimmer needs, and just plumb the drain lines right into the skimmer.
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