Does anyone know if the custom switch (by way of the lens dock) can be programmed to toggle between crop mode and full frame mode?

This should be available in-camera via the menu system. Make sure the lens is attached, and the option to customize that button will appear in the Custom Key menu.

Is this a necessity or is this optional?

It is optional. I bought it but have not used it yet. That said, I anticipate using it since Samyang and Rokinon constantly provide with firmware updates that improve considerably the performance of their lenses (AF speed, AF reliability, etc.) So, if you get rokinon /Samyang lenses with AF this may be needed to get the most out of those lenses. On the other hand, if you are buying MF lenses from Rokinon / Samyang it does not make sense I think to get this.

Where are the fine-tuning instructions to be found for this lens station., for even customer service at rokinon is embarrassingly clueless?

It's the same as with a DSLR. Take a photo of a stationary object with the camera and lens on a tripod, analyze it to see if it's properly in focus, and if not, adjust the fine tuning to correct it using further test shots. 99.9% of the time, this does not need to be changed, as mirrorless doesn't have focusing issues like DSLRs did. The lens is tuned from the factory to focus as close to accurately as possible, and you may see the setting already changed from 0 because of this.

Maximum Aperture	1 f
Minimum Aperture	22
Compatible Camera Models	Sony A7R, Sony ZV-E1, Sony Alpha QX1, Sony A7S, Sony NEX-5R, Sony NEX-5T, Sony A7S II, Sony A6500, Sony A6400, Sony A6300, Sony A6100, Sony A6000, Sony FX30, Sony A7R IV, Sony A6700, Sony A6600, Sony A7R V, Sony NEX-7, Sony A9, Sony A7 III, Sony NEX-5, Sony A7 II, Sony A7 IV, Sony Alpha NEX-C3, Sony ZV-E10, Sony A7R II, Sony A5100, Sony A7S III, Sony A3000, Sony NEX-5N, Sony A5000, Sony A7C, Sony A9 II, Sony NEX-3, Sony A7, Sony A7R III, Sony A1
Lens Mount	Sony E
Minimum Focal Length	12
Lens Design	Prime
Focus Type	Auto Focus
Focal Length Description	12.0 - 135.0 mm
Lens	Standard
Compatible Camera Mount	Sony E
Maximum Focal Length	135

SamyangLens Station for Samyang Sony FE Auto Focus Lenses - Update Firmware Calibrate Focus & Aperture Parameters, USB Connection, Compatible with AF Lenses, Customizable Software

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