For example, if I want to make ads but don’t want them tied to the same identity, it’s easy to make a new account on gchange. Gchange seems to use the same format of identifier-password pairs like the wallets do, so I’m wondering if Gchange will be able to handle multiple keys like Cesium does.
Additionally, how does Gchange prevent Sybil attacks regarding the trust rating?
GChange does not prevent the creation of multiple accounts. Sybil attacks on trust rating are very easy to perform, as is spam advertisement through the messaging system.