Why would Madhu enter the film industry?
I think the story as it is poised gives scope for Madhu's entry into the film industry. Here are the reasons for it.
The gista-pista reason
Trishna had asked Shamsher to request RK to take her in his film. In an effort to fulfil Trishna's wishes, Shamsher may visit RK in his sets and place in his request for a part for Trishna. This only adds fuel to the fire. RK, who is already upset with Madhu for refusing to go with him and presumes that it is because she loves him for his stardom, would think that this only proves that Madhu is after his fame and not his heart. He may vent his anger at Shamsher and sarcastically tell him that he hasn't returned his money because he does not want to take back his daughter and now he wants favours for his other daughter too. Madhu, who comes to the set to try reasoning with RK, overhears this and may be upset with him. Around this time she may get an offer to act and she accepts it because she wants to save Shamsher's honour by returning the money back to RK. Her entry into the film industry also gives her a better chance to recommend Trishna's name to the producers and hope her sister's bitterness towards her is reduced.
A more novel reason
RK-Madhu relationship is unbalanced at the current point. He has a contract which gives him the power to keep her with him till he gets back the money he has spent for Shamsher's treatment. So Madhu has a financial dependence on RK.
They are also socially not matched. RK is a superstar and she is just a common girl from the chawl, a fact that Dipali never ceases to rub in. RK wanted to make them equal by telling her that he was ready to forsake all his fame and wealth as long as she is willing to come away with him and start life afresh. Madhu has no interest in his fame or his wealth but still refuses his offer because she was thinking of both their families. His relationship with his mother is still strained and she had promised her that she will set it right. The relationship between her foster father and her mother is strained because of her refusal to forgive him for what he did to RK. Unless she forgives him, her mother will not be in a position to forgive him. With the impending arrival of her real father, her mother is in danger. She needs Shamsher's protection for which they need to get back together and fight the evil father. So Madhu may forgive Shamsher and urge her mother to do so. At the same time she wants to show RK that she really loves him with all his flaws and not for his fame.
So she may choose to enter the film world to break this financial dependence and social imbalance between them. A career in films would give her enough money to repay RK and also give her equal fame as RK. Without the bonds of a contract to tie her to him or the apparent social divide which may keep her under him, if she still chooses him, it would prove to RK that she is madly and deeply in love with him. The depth of her love will be further evident when he comes to know that she never wanted to be in the industry in the first place and chose it only to bring herself to his level without letting him come down to her level.