How can I quickly heat something to a given temperature with nichrome wire, while keeping the wire itself under a given maximum temperature?
What you are trying to do is usually achieved with two (2) temperature sensors.  


One sensor is in the fluid.  It has a good thermal contact with the fluid, but no direct contact with the heater.
It's used by the control loop to hold the setpoint.  
The second sensor is on the heater itself.  It has a direct contact with the heater.  Ideally, it would be buried inside of the heater.
This sensor is used to monitor for overheat.