As I said it works the same as the overflow box, if in a black out situation the pump stop running and the water level just stay at the same spot, as for evaporation, keep in mind that this design mostly for Salt Water tank so you always need to refill at least 2-3 gallons per day even in regular standard tank, same thing goes for this tank so therefore you must leave an auto top off system hook to the sump and automatically refill for you for a few days.
As to port to hook up to the sump will be located in the black box on the outside by a bulkhead for water to drain to sump, and in the middle of the nano tank there will be another bulkhead for returning pump, so to hide all the plumbing between the rocks.