sure, 10ns are 1ppm, but at 16bit for the first stage and 100hz output the counter input frequency is in the range of 6mhz, or 160ns, so getting jitter to well below 10ns should not be a problem. and analog switches I would not use, would just use a slow filter, no need for fast settling.
similar to how datron does it in their 4808. and the btw used 13 and 13 bit pwms.
that being said, it is cerainly not simple, agreed.