Your Macbook sounds as though it came from the same batch as my wife's Macbook. The Apple store replaced her battery free of charge well out of warranty and also offered a replacement casing. This could be a UK thing as it's pretty clear that there was a period where Macbooks were being manufactured below acceptable standards, the "Sale of goods act" would apply.

I bought a Macbook about a year after my wife and mine has been absolutely fine, no cracking and still with a good original battery.