My frigidaire model FRS26R2AW3 stays stuck in the defrost mode for the freezer way too long resulting in melted ice cream and such. The metal between the doors is hot to the touch. After a day or two it will go back to working normal. Help.

There is a defrost thermostat which prevents the heater from overheating the freezer by breaking the heater circuit when the temp reaches close to 32 degrees F. The entire cooling system shuts off during the defrost cycle and starts back when the timer advances through the cycle.

If this ice is not melted it will continue to build up until the air can't flow over the coil to circulate the cold air through the freezer and into the fridge. The temperature change in the fridge is usually noticed first followed by the freezer.

If the defrost thermostat is bad, it can prevent the heater from coming on OR it won't turn the heater off when it gets too warm. It is clamped to the evaporator coil at the top to sense the temp. If it appears to be misshapen it is bad.
With an ohm meter it should show continuity when cold and none when warm.

The hot metal between the doors can also be caused by the condenser fan which is underneath around back. If it's not running when the compressor runs it will cause the problem you describe. You will need to remove the cardboard backing on the back of the fridge to access the fan.