I like the way you're thinking (I've been contemplating something similar to maximise available vertical height), but having problems visualising exactly what you're planning.
I'm guessing you're looking at something 12V & submersible like this:
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It has suction cups to hold it to the bottom of the reservoir & "run-dry" protection if it detects low water levels.
You'd need to hook it up to a float switch like this to start the pump if the reservoir gets close to overflowing:
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BUT, this is all just a "fail-safe" system in case your F&D delivery volume exceeds the drain sump volume looks like being exceeded?
With a bit of experimentation with delivery volumes & timers, you could have quite the automated F&D system that will never overflow.
Emptying the sump back to the main res. would just be a case of lifting the float valve manually until it's empty.
Am I on the right track?