Substantial turnaround in Rose attitude makes it very clear that she is already at Duke's mercy. Earlier when he was doing her, when they were about to "climax together", he made it clear that he wont flood her because circumstances are not favorable to make them official. He was in control. Last update, while not groundbreaking at physical corruption of her, was quite big in terms of breaking her mentally. She was pumping him, pushing to the edge, showing us how she feel around him. Confident in his obligation, but as the circumstances of kingdom security changes, so are her boundaries. She pushed line further and further, justifying it with well being of the kingdom.
Now all it takes is another excuse, threat from within, wild tribes killing her people... (or as staged event, as we all can predict Ulrod would not hesitate to kill his own men, and thus it is possible to picture event where Leto can be forced to make a choice, either run to wood people and let Ulrod put the final piece in place, coming back with him, and thus, by himself, push her love from the edge), safe day, kingdom needing a strong leader, temporary dictatorship, members of the conspiracy staging a coup.
Within a day, a princess can become a slave to the guardians. Creampied by the duke, telling herself that taking a risk is necessary for well being of the kingdom. But corruption will be complete when it comes from her. Her heart ultimately belongs to "the people". All he has to do is not stop. Just let her take it all.
