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Black point is same for all

Black point is same for all channels in Sony files (and, yes, it is 512 in the file you shared).

The problem is white balance. Here is your image in 'raw composite' view: (not white balanced, green channel(s) is strongest as expected)

So, I see two possible ways:
1) If you'll go 'raw data inversion' way: Invert white balance coefficients too.

2) Generate proper negative image in linear space (so, no raw data inversion, normal processing with linear gamma output), than invert it.

-- Alex Tutubalin @LibRaw LLC