Q: Will packets be unpacked during transmission? Does the service layer need to compose its own packets when it gets the payload?
A: No, the MQTT protocol has a convention, emQ will help to handle TCP sticky packet unpacking. Anyway, just take it out and use it.
Q: How is the payload encrypted?
A: Security, if you consider commercial use, TLS+ business encryption, double encryption. If the TLS private key is not exposed, there is basically no solution.
Dual encryption is important because it is possible to bypass TLS on both Android and iOS.
TLS comes at a cost and adds to the cost.