1- Yes they are born Imams and they knew they’d be the next Imam.
2- They are born with divine knowledge, but it keeps increasing on a continuous basis. So when they are older they have more knowledge.
3- Yes he knew.
4- Yes they would pray but not like the official prayer that later would be revealed. They would pray by prostrating, bowing and remembering Allah. There is no hadith that states they recited parts of the Qur’an before its official revelation.