When + subject + will

2 is possible
1 is possibly possible. I don't know.
O.K. I'll try. If the arrival is a consistent, scheduled event, like at a bus stop, someone might say "I don't know if you have time to go get a Dr. Pepper, because I don't know when he arrives." ( normally )
If the arrival time is totally unknown, someone might say "With the icy roads and traffic, I don't know when he will arrive."
I don't know if he will arrive before dark.