Hi,
1) In mixed mode, the Ethernet physical interface can be either a Layer 2 interface or a Layer 3 interface, but the Ethernet physical interface cannot be both simultaneously. However, Layer 2 and Layer 3 families can exist on separate physical interfaces on the same device.
The following link has more details :-
https://www.juniper.net/documentation/en_US/junos/topics/concept/security-mixed-mode-understanding.html
2) In transparent mode, the SRX Series device filters packets that traverse the device without modifying any of the source or destination information in the IP packet headers. Transparent mode is useful for protecting servers that mainly receive traffic from untrusted sources because there is no need to reconfigure the IP settings of routers or protected servers.
More details :-
https://www.juniper.net/documentation/en_US/junos12.1x44/topics/concept/security-layer2-bridging-transparent-mode-overview.html
3)You should consider using transparent mode because certain networking scenarios are not ideal for a Layer 3 implementation of a firewall.