My understanding is the Royal SRS is providing S.Bus addressing/hub functions, enabling use of multiple non-S.Bus digital servos plugged directly into Royal SRS ports. So, can I use (2) R7003SB Rx's for up to18 channel setup with Rx redundancy?
yes. You can connect normal servos direct to the PowerBox or with the PowerBus to the Bus/PWM adapters
It took me a while, but now I understand - all of the "SRS" capable Powerboxes support connecting the Rx's via the serial link (SRXL, S.Bus, etc.). The Powerbox acts like a S.Bus adapter allowing use of PWM (non-S.Bus) servos when using a S.Bus Rx.
The R7003SB supports up to 18 S.Bus servo channels, so I can use a single R7003SB connected via the S.Bus port to the Royal SRS to support up to 18 servo connections; (2) R7003SB's if I want Rx redundancy.