Just playing devils advocate here BUT if sellers have OFFERED her refunds then its not feedback extortion & I'm even wondering if it qualifies as feedback abuse under ebay guidelines.

However,IF she has said the exact same thing as she's said to you

I respond by offering her a full refund. She then responded by saying she will remove her negative feedback if I give her the refund which I then did

THEN you need to contact ALL of the sellers & hope they will join forces with you so to speak & email Trust & Safety.Good luck with any asian sellers though as they may not understand or particularly care.