Keeping in mind that I don't know the stories of the Vedas and Purunas, here are my comments why bringing Arunasur's mother back to life was against the Laws of Nature however it was not so when Nandi was brought back to life, or when the older fellow was turned into a young man and the other young man was turned into older man:
Arunasur's mother has been long dead and therefore her soul is no longer connected in any way to her physical body. Nandi's body was only dead for a couple of minutes before Mahadev brought him back to life. His soul was still connected fully to his body. It is common today to hear of people who have near-death experiences where they are physically dead however their hearts do again start beating and they continue to live normal lives. They live to recall their out of body experiences and tell how they saw a crystal or silver cord connecting their souls to their physical bodies. For Nandi, Mahadev merely had to breath the breath of life into him. All his soul parts were still connected to his physical body. It wasn't yet his time to die.
In regards to the old man and young boy, they were never dead and their souls had never departed or disconnected from their physical bodies. The outer appearance is only an illusion and any true Master especially a God such as Mahadev, can altar the appearance of the illusion from within our viewing or upon that which is viewed. In regards to their desire of experiencing the events at either a young or old age, time itself is an illusion. A Master can take the soul to any point in past or future at any age of the physical body.