If you can't locate an external leak (No coolant anywhere on the ground), then your mechanic is wrong. There's absolutely no way that your engine can be using coolant unless the head gasket is blown.
If your coolant is disappearing, then it has to be going somewhere. There's only two places that it can go.
Depending on what part of the head gasket blew, it can go into the oil or the exhaust pipe. Pull the oil dip stick. If the oil is white (looks like a milk shake), then your coolant is getting into the oil.
If the oil good, then it's going out of the tail pipe. One way to double check: When your engine is cool, take off the surge tank cap.
While someone revs your engine to at least 1500 r.P.M.'S, look into the surge tank. If you see air bubbles, then that is exhaust gases getting into your coolant.
This is what's causing your overheating. Because your system is what we call a " Reverse flow" cooling system, the air bubbles get trapped into your heater core and radiator. At idle, the water pump is too weak to push the coolant past the bubbles. But as your engine speed increases, it'll push the bubbles out of the way.
Another way to verify this, turn your heat to full " High" when your engine is completely warm. If it is blowing cold air at idle, but blows hot when you rev the motor up, then that is proof of the air blocking the heater core.
The reason (I believe) you mechanic misdiagnosed your engine problem: He put a compression tester on your engine to check the head gasket.
But with you particular type of motor, the gasket will only leak when the engine is warm and under a load severe enough to cause the cylinder heads to pull away from the block. (It can only happen while driving)
It's isn't possible to check this condition with normal service procedures. Only someone familiar and experienced with this problem can accurately diagnose a blown head gasket on a reverse flow coolant system.
I hope this information helps. You may want to print out this message and take it back to your mechanic. He/She can verify my information.