1. No, change of status credits are not applied to class S Prsi.
She would need 1 paid class A contribution to trigger the change of status credits.
2. If she gets the 1 paid class A she could pay voluntary Prsi based on class A instead of class S. Class A voluntary contributions are still 500 euro per year, if she has minimal class earnings the previous year. She could make voluntary class A contributions starting on the next full calendar year after her retirement year at a cost of 500 euro per year.
For this to work she would probably need to avoid gaining class S until after she plans to stop voluntary contributions.
She could also do as you state by lowering her ARF to below 5000 euro per year and then paying 650 euro per year. She could also restart class S at any time after paying class S voluntary contributions.
Voluntary contributions can only be made up to age 66. After age 66 she would need class S from her ARF or paid class A from employment to increase her Prsi record.