1998 Cadillac Catera Question

My question is:  If my initial complaint was: Oil mixing w/water & running hot.  Was the issue misdiagnosed & should this be under warranty.  Here is the issue:

Estimate stated; $2700 for Head Gasket Job. I paid the $2755.65.  Receive a 12 month warranty on the repairs. Next day oil light came on… Got a $60 estimate for an oil sensor… 2 1/2 weeks later… the oil is mixing with the water again. We take it back again, this time an $1130.83 estimate for an engine cooler.. Repair shop says not related to $2700 issue thus not covered under warranty.