Modeling ADC in Active Noise Cancellation
I am also working on ANC, with my understanding may be I could provide some clarity for you.   

I could not see your point exactly on ADC modelling, but as far as I know this ADC transfer function will be covered in secondary path modelling.


  "However, there is also the transfer function from the noise source to
  the adaptive filter."


Do you mean modelling of the reference input microphone, as far as I can see only transfer function involved between noise source to adaptive filter is reference input microphone function.  

The way you are using secondary path description is also a bit confusing to me. 

But here is what I know from this paper page 945


Here P(z) is the unknown primary acoustic path between reference input sensor and error sensor.

Below text directly quoted from the reference provided


  Summing junction represents the acoustical superposition in the space from cancelling loudspeaker to error microphone, where the primary
  noise is combined with the output of the adaptive filter.   
  
  Therefore, it is necessary to compensate for the secondary-path
  transfer function from y(n) to e(n) which includes the digital-to-analog (D/A)
  converter, reconstruction filter, power amplifier, loudspeaker,
  acoustic path from loudspeaker to error microphone, error microphone,
  pre-amplifier, anti-aliasing filter, and analog-to digital (A/D)
  converter.


The compensation as you might have already known is done by offline or online modelling techniques.  

If you want to include your primary path modelling as well i.e reference microphone TF, you can do that or if you are considering feedback effect, then again in the feedback compensation this reference microphone transfer function is included.  

Please correct me if I am wrong.
Thanks for bringing this up!