He was so sure that Xue Yan and Jiang Yue would protect Empress Dowager Minde outside the palace because they would learn about Fu Ying's weakness. So, as long as Jiang Yue and Xue Yan heard that Empress Dowager Minde would leave the palace to pray, they would immediately think that Fu Ying's mother, Empress Dowager Minde, would fall into Fu Xu's hands. Thus, they would want to protect Empress Dowager Minde.
Empress Dowager Minde has always been a stubborn old fogey. When the time came, the possibility of her dragging her daughter down was naturally extremely high.
As for why he said he wanted to capture Xue Yan and not mention that he would kill him, it was because Fu Xu wanted to capture Xue Yan alive and use him to threaten Da Ling. If he killed him directly, it would not benefit Fu Xu at all. If Da Ling knew about this, they would immediately send out troops to flatten Beicheng. Fu Xu definitely wouldn't want that to happen as an Emperor.
