I am neither a teacher or a native English speaker. I just try my best to touch it up.
To be frank, I don't think your points are well defensible enough. Here's my two cents.
- I'd like to suggest you use indisputable factual evidences to illustrate how your daughter was treated unfair rather than words only.
- It is better to use declarative sentences rather than interrogative sentences. For example, my daughter had to often work overtime as the work-time card recorded.
- I can't understand why you say "I am ashamed of being a Chinese" in your letter above.
- You don't mention your intention in the letter. For example, you are demanding an apology or financial compensation.
- Why don't you let your daughter to write the complaint letter, at least write under her name, since she is an adult?
- It is not good to denigrate the company your daughter worked for though she has to leave now, especially the letter is for the managing director of its own company.
I don't think you are really glad for that. Obviously, you are angry.I am pleased my daughter Yeung W Y has left a financial organization, which is dishonest and is run by an unprofessional management.
So, it might be better to collect and have enough evidences stated in your letter before sending it out.
I hope other forumites or teachers could give you a hand.