One kind of curious thing is that if my understanding is accurate, the 37C65 already has the functionality of the 92C32 built into it, so why the separate 92C32? I suspect that the VCO and the XP9103 are in some way used to control the motor speed of the floppy disk.

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Sort of. The 92C32 and the VCO are there so the card can set an arbitrary bit rate (the 37C65 only supports the standard FM/MFM rates). That way, it can read 800k disks without changing the motor speed; PC floppy drives are built like HDDs, and are locked to a certain speed (usually 300 or 360 RPM). It's also, most likely, using the XP9103 as a cheap substitute for a DAC (I don't have a have data sheet handy for it, so this is pretty much a WAG...) to control the VCO.

The Deluxe Option Board actually uses a similar trick, though they used a UMC data separator instead of a Western Digital.

-lee