Cardholder to Merchant SET Protocol
In SET, Bob the merchant doesnt need to know message is from Alice the cardholder. Bob needs to get paid for the order received. So Bob proceeds on response from Acquirer (running a SET payment gateway) telling him this particular cardholder (whatever the name) has sufficient funds in his account. This would likely mean funds were reserved, according to policy of issuing bank.

Yes, a "dual signature" was introduced in SET, meaning message to be signed is actually just two hashes, one is the order and the other is payment information. Merchant is supposed to receive the order and the hash of payment information, unless gateway would open more details.