SPOILER! SPOILER! SPOILER!
The emperor suspected an affair between Zhen Huan and Prince Guo, so he sent her to give poisonous wine to him. If she could do, then she proved her innocence. Faced with this dilemma, she went to see Prince Guo with the poisonous wine. However, she had no intention to let Prince Guo drink the poisonous wine and actually decided to drink it herself to save him. Prince Guo always knew that the emperor wanted to kill him and felt odd that Zhen Huan would seek a private audience with him. He sensed something was amiss and figured out that the wine that Zhen Huan brought was poisonous, so he switched the wine cups and ended up dying for her.
Regarding you questions...i didn't watch the whole series yet, so i can only answer a few of your questions
Ep.56 - Zhen Huan returns to the palace