Your correct, tongue weight and tow capacity are two different things but they are related. Typically you want the tongue weight to be about 15% of the trailer weight. So 85% of the trailer weight is on the trailer wheels and 15% is on the tow vehicle hitch. This provides stability while towing. Too little tongue weight and the trailer becomes unstable and will start swaying back and forth. This can become severe enough to cause loss of control. Too much tongue weight can lift the front of the vehicle resulting in loss of control.
The maximum tongue weight is 360 pounds for a Pacifica with the factory installed tow package and 149 pounds without. Yes, the load in the cargo area must be considered. If the cargo carrier obstructs the view of the rear lights or license plate then, legally, you would need to add lights and plate to you carrier. I would try to get a carrier that is low enough not to obstruct the lights or plate.
2017 Pacifica Limited (built May 2016)
Vacuum delete, KeySense and Tow