2005 Nissan Altima Head Gasket

Last winter, my vehicle's heater was intermittently going in/out. My car also seem to struggle to start. We replaced the spark plugs and wires. Then, the vehicle was diagnosed as having a blown head gasket, as water was found in the oil? The car had 60, 900 miles at the time of the diagnosis, which is just 900 miles over the warranty. Therefore, Nissan would bear any responsibility in assisting in the repair despite the fact that the initial problems occurred prior to the vehicle turning over 60K. Anyhow, we had head gasket repaired for $1200 and the vehicle was returned to us 2.16.09. On 12.14.09, we sent the vehicle back to the mechanic who repaired the car in February and were informed that the head gasket needs repair again, but now I believe the problem is in all cylinders? The vehicle NEVER overheated, not the first time and not this most recent time. However, the mechanic is alleging to my husband that he wanted to take a closer look at the lower half of the engine and my husband discouraged him (in February). We consulted a Nissan reapir man who stated that would be unnecessary, as he should be able to see the lower part of the engine when the head gasket was removed for repair? Anyhow, the mechanic is now wanting to charge us full price again to repair the vehicle and appears to not want to take any responsibility. Please advise. Thanks. :)