About the "Hair"...
On one hand, if you both write down the "Short Hair Needed" in the contract, she must follow it. It's the goal of contract, right. Talk about it with her in peace. Try to make a neutral agreement again with her, otherwise you have a right to terminate her contract as she break it out.
On the other hand, if she can tide up her hair during working hours and keep her hair hygiene good, I could not see any problem of it. What is your concern about her hair? Think about it.
About "underpaid"....
This is also a WEEK POINT on our side. Hong Kong Government starts to prevent this happen again and she force to encourage those domestic helpers whose salaries are underpaid to sue their employers. Take care about this event. [only my personal opinion.]