Well for the first question of what “Delta Segment” is, Google search led me to a (what seems to be a) non CommonVoice page here:
The files entitled “Delta Segment” consist of any additional audio files that were added after the main dataset was created. This means that they are basically updates that allow you to download them as they come in, without having to download the full dataset again.
It was the fourth result on Google search (What is “Delta Segment” in CommonVoice). I wonder whether the problem, is Google Search or Inadequate Documentation on the part of CommonVoice. I am still interested to find if there is any mention of “Delta Segment” from an official CommonVoice page.