By law, she has the right to stay in Hong Kong for 14 days, therefore, to play safe give her a ticket on the last day, i.e. if you fire her on the 1st give her a ticket on the 14th? (you better call up Immigration to double confirm).
Just in case, I mean just in case... I issued a one year open ticket for my ex-helper because I knew she will file a claim with Labour Department. As long as you are sure that she is not going to play any tricks or file a claim with Labour Department, you could give her a ticket (14 days counting forward from the date of termination), otherwise you will have to responsible for issuing another ticket for her.
Once the contract is terminated meaning the relationship between you and herself is ceased, she cannot 追討留港期間的住宿費??
Let me know if you need further clarification.
Regarding 大假, your agent is right, at least this is what I have been doing. Regarding legal concerns you may call up 24-Hours Government Hotline 1823 than ask to talk to representative from labour dept of Immigration Department and you will be able to get instant clarification regarding your concerns.