By no means am I an expert on the maths behind this, however… The more appropriate way to describe the modulation stage would be as a “noiseshaper”, as the audio community is pretty firm on the definition of delta-sigma modulation with one specific implementation.
That said, the key aspect to consider is that even with DSD running at the same rate as the Ring DAC with only 1 bit, you get close to 24-bit performance. We are working with a much greater word length here, and while it really isn’t an apples to apples comparison (DSD to the Ring DAC format), it stands to show that a lower bit depth and higher rate – if handled correctly – can produce excellent results.
It is separated, yes – one side of the DAC board handles one channel of the audio. This avoids crosstalk between the left and right channels.