1) In our image that is the shutdown button. It will shut down pi so you can unpower safely.
2) Maybe
@waitForAllTemperatures maxDiff
Will wait for all set temperatures to be within the given difference. If maxDiff is omitted, the last or default value gets used. This will only prevent the running job from being executed while manual commands can still be send!but problem is off is 0 which you will not reach so make maxDiff something like 40. Only useful in a automatic shutdown script.3+4) Thats the problem. You can use then alexa to switch down main power but since pi is not running it can not do that. In other cases you would need some electronic with delay time to switch of main psu.An alternative would be if there is some other always on device you can program. Then in shutdown script you could call that computer to disable switch with a delay.