Hi,
We would think that if you wanted to use S\PDIF to connect to your Lexicon hardware reverb that the I/O setup should be stereo, not mono. If you have S/PDIF I/O on the Focusrite and you want to dedicate it that, create a stereo out and assign it and also create a stereo in and assign it, in the Audio I/O tabs.
We'd also assume that if the Lexicon needs to clock it would take it's clock from the S/PDIF input, if there are internal settings on it to determine where it gets it's clock.
Also, if it has Midi In you also may want to look over it's Midi Implementation in the manual as you may be able to create an external device for it to control it's settings without manually operating it, to load a song that uses it and have the settings come back from automation maybe.