Exhaust leak in the cab when blower is turned on

Most likely the gaskets need to be replaced... maybe the valve cover, oil pan, etc...if oil gets on the exhaust it can cause the oil to smell

Go under the car and check for signs of fuel leaks, you will see very obvious stains if fuel is leaking from the system. I suspect you are smelling diesel exhaust.

EVAP leak

smell the exhaust after you fill it up and see if it's burning in the engine if you smell the antifreeze it's going into the combustion chambers it could be a leaking gasket or a cracked head or engine block if not look around the radiator/hoses or the weep hole under the water pump your leaking from somewhere if you don't smell it in the exhaust

Quite possibly the exhaust doughnuts or the exhaust may be leaking

I can answer some of your questions without being there in person.
The smell inside the van is from fresh air entering the venting system. Not sure why you don't get it from all vents. The fresh air intake is in front of the windshield and will pick up any smells from the engine compartment.
Loosing the serpentine belt would not necessarily cause you to loose ps fluid, but having a leak could drain the pump and damage the bearings in the pump, causing the pump pulley to drag on the belt.
The ps pump could be in the final stage of failing.
And the shaft seal in the pump could be leaking fluid onto the engine and exhaust.

if inded the smell of exhaust is getting into your car i would suggest not driving it until it is properly checked by a licensed meachanic. it could be entering through your heater/AC blower and in return be fatal if left untouched for a period of time

You could have an exhaust manifold leak and it is just seaping up through the cab. Just start the truck up and jump out and see if you can hear or smell the exhaust coming through the wheel wells. It may be tough to hear but just listen for leaking air thats abnormal. Hope you can figure it out..

Check door rubber seal for damage / wear.

To test exhaust simply place a flexible object over exhaust pipe to seal it. If no leaks it should blow it off.

1 Inspect OXYGEN Sensor Damaged or Faulty OXYGEN Sensor or Circuit. 2 Inspect Timing Specification Incorrect Ignition Timing. 3 Inspect Catalytic Converter Clogged or Faulty Catalytic Converter. 4 Inspect PCV Valve Plugged or Damaged PCV Valve. 5 Inspect Carburetor Dirty and Improperly Functioning Carburetor. 6 Inspect Hose (PCV)at Clogged or Collapsed PVC Hoses.