If the Meng Family is truly a pawn of the Crown Prince, then this suggests that the Crown Prince tacitly approves of those pirates invading his own country. But the Crown Prince isn't foolish—doing so would only push himself onto a path of complete betrayal and isolation, which is absolutely impossible.
That being the case, the Meng Family must be misleading others—or more specifically, misleading the Imperial Court or even the Emperor.
Then what about Prince Yun? Could this be Prince Yun's handiwork?
Although Yang Ruxin wasn't very familiar with Qi Jingyun, she had interacted with him several times. Setting aside personal prejudices, that man was capable, and indeed ambitious.
Moreover, his mother now holds the Phoenix Seal in the harem. So, anyone aligned with him would naturally want the Crown Prince to fall. However, she didn't believe that man would stoop to conspiring with outside forces to invade his own country.
