I also use submersible pumps for my returns. My sump is not drilled.
I skimmed over the responses and questions you have asked, I would not suggest drilling a hole in the tank for your return line. I understand you wanting to have everything nice and neat but keep in mind that the return will back siphon water when the pump is off.
As long as you match the pump flow to the drain you will be fine on water flow. A 1" bulkhead can handle about 600gph via 1" PVC
. I would think a Mag7 would work just fine as your return pump.