1) The time for each is before the respective obligatory salat.
2) The time for establishing dhuhr and asr can be achieved by looking at the shadow of a stick that is vertically placed in the ground. As the shadow gets longer the time is indicated to be progressing. Indeed the time for those nawafil is short when considering the actual start of the obligatory prayer period. But one can pray them nonetheless and then commence with the obligatory prayer.
3) You are never not allowed to pray nafilah, you can even pray it’s qadha. The only exception is if you have not prayed an obligatory prayer and it’s time is running out