It's more accurate, I think, to say it's proof that God wants this servant of His to be honored, since it's God who actually wills and does the healing or whatever the miracle was.
I belieave the souls in Purgatory do hear our prayers. But can they intercede for anyone — resulting in a miracle such as the one accredited to Pope John Paul II.
I’m basing my statement about hearing the prayers while in Purgatory by stories from Maria Simma’s “Get Us Out of Here” book on Purgatory.
The souls that were prayed for go to meet those people (newly deceased) who prayed for them while they were still suffering souls in Purgatory.
A most interesting book by the way, and I’ve read only part of it.