The USB is for USB Virtual Com Port (on the other USB connector) - it's a bit faster than the standard Serial port connection. There's no OTG though - I don't think F103 chips are even capable of it.
Not sure why you had the trouble with SPI1 - sounds strange that the backlight would dim - maybe it's on one of those pins? The board config could just be wrong and you could update it to 3 SPIs and try that - but if SPI2 works it's maybe best just to keep it!
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