The suggestion of the 75Ω resistor is to terminate the output properly so as to avoid standing waves. But there are some assumptions - first, that the RCA S/PDIF output is really 75Ω, and second, that the input impedance of the Focusrite's RCA S/PDIF input isn't already 75 ohms. Given that you're talking about devices that are 25 or more years apart, it may be that the output level from the Fairlight is just too low to drive the input of the Focusrite.
What's the Hosa adapter? Coax to optical? When you're sending data (or think you are) from the Fairlight to the adapter, does the optical output light up?
Have you checked to see if you have the Focusrite set to the same sample rate that the Fairlight is sending? They have to be pretty close in order to sync. If the Fairlight is set for 44.1 kHz and the Fairlight is sending 48 kHz, that might be your problem right there.
You're correct that the RCA S/PDIF jack should be 2-channel audio, but Focusrite makes a bunch of interfaces with an ADAT optical input so that you could connect an 8-channel mic preamp to it to expand the Focusrite's input capability. Only a few products that use ADAT optical for expansion have a switch that converts them to Toslink S/PDIF.