The mechaninc's steps were logical and good. The only question is if his test results are reliable. A cracked block is unusual, but so is a failed oil cooler ( with only 90K miles, and we know you were using the correct, corrosion-limitng cooolant). I think the cooler is more probable, though.
If you can pressurize the cooling system (not including the oil cooler), you should force coolant into the oil and determine if the leak is on the engine side. Ther's usually special equipment for that though. See example
here