I'm not sure if it's just my speculation. But I think ex-fiancรฉe was instructed to act weak before ML. Princess…
She is clearly a tool to drive that wedge between the lead couple (not to say that the FL will also do it through her actions) but I donโt think she is the princessโ tool but rather Marquis Muyang and Consort Qin because they have realized how strong an ally Ye Li is to Prince Ding.