The test's that Matt did to come to this conclusion was a leakdown test on the cylinder's, less than 5% on all of them, eliminating head gasket. A dye in the coolant to detect a cracked block or cracked head, nothing there either.
Yeah, it could be a bad cooler, but still don't overlook the head gasket. Just because the cylinders are not leaking by, is just one type of potential head gasket failure. Remember, you can have the following types of head gasket failures on newer OHC motors: (1.) Pressurized oil port into water port while running (Becomes pressurized water leading into that oil port when engine is turned off if cooling system is pressurised at that time). (2.) Pressurized water into oil drainback port. (3.) Oil port to cylinder. (4.) Water port to cylinder. (5.) Cylinder to Cylinder (6.) Cylinder to outside of eng block. (7.) Water port to outside of eng block. (8.) Oil port to outside of eng block. (9.) Some or all of the above mixed.
My point is that just because the leak isn't from a cylinder to a water or oil port, then how do you know, because the head gasket is sealing many ports simultaniously
As far as the helicoil in the head goes, A PROPERLY installed sparkplug helicoil should last the life of the vehicle.