Since Evren's mother was royalty and the next queen, her people hide her whereabouts including her coronation. They would have rather crowned her brother, but his father made it clear who he wanted to rule, even if it was clearly a woman.
At this time, Evren was already 12 years old, he was now old enough to know and understand what was happening around him. He was old enough to understand how scheming and evil Iris was.
The Bancroft's took him away after the news of his mother's death even if he knew she wasn't dead. It was all part of the plan. He needed to be away to exact his revenge for the things they did to his mother.
Evren returned bacause he wanted them to pay for the humiliation and pain they caused his mother. He wanted to see Isla suffer and beg his mother for forgiveness, especially after she openly told him she had hurt his mother and would kill her again if she had the chance to.