2) Any how she is LOP so even if it is dated Feb 2017 the revised salary may not be paid. So anything is fine
Jeevarathnam P sir is saying the same thing. Employee is on LOP. So even if you implement the appraisal from Feb 2017 she wont be paid anything untill she resumes the work in the month of April 2017.So ultimately she will be getting her revised salary in the month of April 2017 only.
It's important to know that salary increment is not by product of a performance appraisal process. Performance appraisal is a much larger ball game. As a practice companies link the PA with merit awards and salary adjustment.
If the PA is not related to salary adjustment, feel free to do it when she joins back and resumes her duties. Talking about salary adjustment, it is vital for you to first refer to your HR manual to assess how many days of continued presence is expected from an employee to be eligible for a salary adjustment.
If she is eligible for a salary adjustment, let her resume her duties and perform PA to follow the next course of action. She can be paid arrears, if the process gets delayed for her. Please feel free to drop a mail to her expressing your inability to do a PA and clarify to her the process you will follow after she resumes. I think that would help you manage expectations at both the ends.