Paulster is right that coolant could be leaking into the cylinder rather than into the crankcase. This would explain no milky dipstick. But your mechanic said you only had a pin hole leak in the head gasket so I'm wondering how much coolant could leak out of such a small hole and cause so much coolant loss.
One idea I have is that perhaps your water pump is defective but it's not completely shot. I'm not sure if there's a possibility of partial loss of effectiveness for a water pump, but my idea is that when you drive fast the pump is able to move enough coolant through the cooling system AND higher speed air through the radiator cools that coolant flow more efficiently. When you slow down, the pump slows and pumps less volume and the radiator doesn't cool at slow speeds as it does at high speeds. Higher speeds may enable the pump to also pump water through your heater which has an additional cooling effect as you know. Not so at slow speeds, therefore no heat from the heater and insufficient coolant flow through the system leading to overheating.
An overheating engine would cause water to be pushed past even a functioning radiator cap. Seeing the coolant around the air intake as you did may be what happens when you've had coolant loss and this is what was blown out after you slowed down at the end of your trip.
In my experience it doesn't take but a couple of miles driving when the engine is warm to get very hot coolant. I think you're losing a lot of coolant through poor circulation. A pin hole leak in the head gasket seems just too small to vent that much gas into the cooling system but wiser heads may know differently. Pressure in the cylinders is very high, of course.
The presence of coolant inside the engine compartment is an important clue. I bet steam pushes that quantity out not venting exhaust from the supposed pin hole leak.
One thing you can try when the engine is cold and its got all its coolant, is take the radiator cap off and start the engine. After the thermostat opens, you should be able to see some current flowing at the top of the radiator. If it starts to boil over and you don't see that flow, that might point the finger at the water pump. If it were a head gasket leak, I'm thinking you'd see that in the form of bubbles after the thermostat opens. You'd see bubbles, current and then boiling (because of unpressurized system).