Hi,
I might be mistaken but I have thought to understand the use of the optocoupler to disconnect the +lead between the receiver and the ESC to avoid problems with different voltage levels. It is not to be seen as a galvanic disconnection. As a result minus is ground level for the saver as well as the ESC and the receiver.
I don't get the idea of "sender" in your discribtion.
Regards
profimc