He father knows that the stepsister is the daughter of the pawn shop dude, and this guy was his deputy. He is definitely behind this to control Shanghai and take away the power from ML's dad.
Yeah she wanted to get the urn back, as well as take the place of the SML.
Yes you are right, and it depends on the perspective. Generally, she made herself a better reason to be held hostage claiming to be the fiancée as well as the richest man's daughter. With that her dad will also try to convince and also contribute to the army making it easier for them to make the telegram. However, even if the SML is taken hostage still the telegram will be made eventually I think... but not the urn and the connection to the richest man.
Though he is quite poor thing... too bad his dad is a jerk.