2006 BMW 325

Engine Mechanical problem
2006 BMW 325 Automatic 75,000 miles

I have a 2006 bmw 325i and have replaced 4 of the 6 engine coils. About 2 weeks ago I noticed at start up that the car idled rough for about 3-5 seconds then was fine. No service engine light. I assumed that one of the older coils was going out. Just last week on start up, the car idled hard and stayed there. Now the engine light came on. I took it the my machanic and he ran a diagnostic test. He said it showed #3 was out. When he went to replace it he said nothing happened. He said that both the #3 & #4 were out-- although the diagnostic test didn't show this. He said that there was oil in the engine coil compartment and that both were "dead holes." He said that I have a blown head gasket and he thinks a potentially crack head of which will cost me almost $10,000. But My car has never over heated and gets regular oil changes-- every 5000 miles. How could this be possible?