To answer your questions:

Q1. What are my obligations to her as an employee?

You need to investigate the reasons for absence and the likely length the employee may be away for. Check to see if there is anything to do to assist with her return to work. Keep in touch with her on a regular basis to get updates on her condition but do not go over the top so as not to be seen as harassing her. Keep her in the loop about work practices and changes

Q2. If she decides to take an extended leave of absence what is the guideline?

She will technically be on sick leave,. You may allow her to be off sick for as long as you want and just keep paying her SSP for as long as she is eligible. It is up to you whether you allow her to be off sick indefinitely or if you wish to consider terminating her employment

Q3. If she decides not to continue working what are my obligations?

You have to consider if she can be retired early if possible. If not then you can simply terminate her employment on grounds of capability or if she resigns - just treat this as a resignation.

Q4. She was scheduled to take annual leave in two weeks time. How do I handle this if she is currently unable to work due to health reasons?

If she had pre-booked holidays and then went off sick she is entitled to have that leave carried forward to a time when she can take it.

I would be grateful if you could please rate my answer and provide quick feedback. If necessary, I can then provide further advice and guidance as required and answer any specific questions you may have. Thank you