16psi is fine on a radiator cap, that's what I'm running. A few lbs either way won't kill a head gasket, and if it does, it was about to fail anyway.
Your heater and hose symptoms and are consistent with a leaking head gasket. Ensure the radiator is full of coolant with no air pocket on top. Warm the engine up to operating temp, open the coolant overflow tank, and look for bubbles. If you get bubbles, you have exhaust leaking into your cooling system.
When dealing with the shop, put a boot up their backside about whatever they did that is leaking now. The head gasket is most likely unrelated.
Edit: Are your radiator hoses pressurized when the engine's warm?
Your heater and hose symptoms and are consistent with a leaking head gasket. Ensure the radiator is full of coolant with no air pocket on top. Warm the engine up to operating temp, open the coolant overflow tank, and look for bubbles. If you get bubbles, you have exhaust leaking into your cooling system.
When dealing with the shop, put a boot up their backside about whatever they did that is leaking now. The head gasket is most likely unrelated.
Edit: Are your radiator hoses pressurized when the engine's warm?