There is an error in dacs.json file - it specifies wrong driver for “PI 2 Design 502DAC Pro” - hifiberry-dacplus. Only driver in list that is compatible is hifiberry-digi-pro. Its not critical but misleading. PI 2 Design 502DAC Pro is based on WM8804 I2S master with two clocks switched by GPIO5 & 6. As it seems proper driver that used PCM1792 hardware volume control trough I2C was never developed 
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careful, that may not necessarily be an error in the cards.json profiles but a problem with the card identification from 2 different boards. We need more info to investigate.
dacs.json file clearly states identical drivers for “PI 2 Design 502DAC Pro” and “PI 2 Design 502DAC”, this can not be right because dac without pro is based on PCM5122 chip as I2S master and clock switching is done by GPIO3 and 6 on DAC chip itself. If I set manually PI 2 Design 502DAC Pro I see on I2C bus that Raspberry trays to contact wrong address at startup and after that says that hardware is not responding. I have oscilloscope with serial decoding and can check that. This is how i first spotted it. I hoped that setting to “PI 2 Design 502DAC Pro” instead of “Hifiberry Digi Pro” will give me hardware volume control.