Yes that new circuit is what I meant.
The gate pull down resistor (the resistor's end not connected to the gate) and the source and the -12V all to the same point just like you have.
Can the +12 to optocoupler collector be avoided? Because +12 has 30 A.
The fact that the 12V supply can provide 30A should be of no concern to you nor the transistor. The current drawn will only be in accordance with Ohm's law, and if that doesn't dictate 30A then 30A will not flow. The important thing is that the 12V supply is totally isolated from the arduino circuit.
is it save to solder the 4n35.
Does the high temp destory the IC?
Only if you take more than ten minutes with the soldering iron on it.