It should be ok. Many of the helpers have signed but did not show up. Understand some of the agents told employers the one they signed refused to take the job. Then, why not.
If you do not provide her the release letter, how can she proceed the visa application with HK Immigration?
So, if both of you want to keep the relationship, just ignore the agent. Even the application is being processed, both parties still have the rights to withdraw.
Sometimes the helpers are being convinced by the agents who want more business. If the agents do not encourage the helpers to leave, the suffering employers won't need their service.
This would have a very bad effect - the helpers would think they are very "hot", picky on jobs. I think the government should take some measures to stop this, e.g. compulsory return to home country for those breaking contracts; interview requirement for visa application...
1) If she can resign once, she can resign a second time later. Can u trust her?
2) Ask her to call the Immigration Dept herself if she does not trust u. There are many "no shows" with no consequences. Even if the maid gives you a resignation letter, until you sign to acknowledge receipt, it is not a valid document for Immigration without your signature on it.