"Tap redeem..." not sure exactly and it likely depends on a few factors: which ecash wallet, which nostr client if used, but I did it yesterday. Someone sent me a generous tip, it appeared on Amethyst as a message, I tapped redeem, the sats immediately appeared in my Alby wallet (yes, I know, Alby is custodial too...things likely can to be set up to go to whatever wallet is preferred). I'm not sure the mechanics on your question about unsolicited redemptions? My guess is that the mints trust the Cashu token is truly backed by sats. By "trust" I mean that the mint knows that the cryptography behind the protocol ensures that any ecash token is legit, therefore it's accepted.

I can appreciate your skepticism...that's the bitcoin way and it's healthy. I think most of us are still learning, I certainly am. Maybe someone out there can help me out on the question of whether mint B will redeem a Cashu token from mint A and get me from 99% sure to 100%.