I was thinking today, I don't think Trump can arrest Hillary until the last half of his second term. Arresting her will be the biggest threat to his life and his family's safety that he could do/will do. To continue to close the walls around her while she is still out is advantageous both as psychological torture to her and her friends and also to campaigning.
I'm not saying they don't have the power or the evidence to put her away tomorrow, but realistically if putting her away is a huge threat to POTUS, he has a lot of high priority things to get through before going through that threat.