Ecash "on your device" are just claims to sats on a mint.

MINTS VALIDATE IF A TOKEN WAS ISSUED BY THEM AND HASNT BEEN SPENT, THEY DO NOT STORE THE TOKENS ON YOUR DEVICE, YOU STORE THEM HOWEVER U WANT

The mint is connected to an underlying lightning node. The mint creates ecash tokens that are redeemable for sats assuming the operator of the node doesn't withdraw them before you redeem your ecash tokens. There is nothing else you can do to prevent them from rugging the sats backing the mint.

Normalement c'est ce qui doit se faire lorsqu'on évoque Btc et non l'utilisation parallèle d'un système LN pour juste changer du système Fiat