The diode on the power switch? this will half rectify the mains to the power relay, i am assuming it is meant to be in anti-parrellel with your optocoupler diode behind the resistor.
You could do what most other smd stations do, have a reed switch in the holder, when the hot air nozzle is placed in the holder, turn off the heat, and keep the air running until it is cool enough for your design,
What you are describing is not an auto-off, but rather a delay off, to accomplish this you only need to control 1 relay, when the device is turned on (detected by the optocoupler) you switch 1 relay, you have normally open connected to live, normally closed connected to the switch output, and common connected to the devices switch output
You have a second relay then connected so while the switch is on the heating coil is connected, you switch off the switch, the heating coil is disconnected.
By doing so, the device no longer turns off when the switch is flicked, because the relay holds it on, the other relay breaks the heating coil connection, and its left running air while no heat is applied.
To better accomplish this, you will want to tweak how the optocoupler is wired, instead it should be between the switch output and ground, this way when its switch on it pulls low, and with the relay switched, If the switch is released, it stops pulling low. you then either wait X seconds for it to cool enough, or just measure it.
using appropriatly rated resistors (you may want a few in series for a voltage rating),