a) Yes, the connection cable is included.
b) It is approx. 95 cm long.
c) It is designed so that a second cable (of the same length) can be used to connect a second battery stacked on the Delta Pro 3.
Unfortunately, according to the manufacturer, the Delta Pro 3 auxiliary battery can only be connected to the main unit via the Delta Pro 3 auxiliary battery cable. It has a proprietary connection. The Delta Pro 3 additional battery cannot be used with the Delta Pro.