The best encouragement I can offer in response to your plea for the Nemeth Code to provide us with a specific rule for unitemized main entries with subdivisions, such as a bulleted list is this: The Nemeth Code is not an independent code meant to cover every aspect of a textbook publication. We use all of the resources available to us regarding a literary braille transcription in addition to those that specifically apply to technical works using the Nemeth Code. Some Nemeth formats or rules supersede those followed in a literary work; those not mentioned rely on the literary codes to supply us with rules and suggestions. Your consistent application of whatever format you decide applies will produce an excellent transcription.
A math/science book is not entirely written in mathematical symbols. It contains narrative, illustrations, tables, and --yes-- bulleted lists. As a transcriber your concern is to present the material in a manner which will be consistent so the reader can discern an order that should help with the comprehension of the material. Formats, in any code, are developed in order to attain this goal. The most recent revision of the Nemeth Code came to us in 1972. Its rules have survived three major revisions in the literary braille codes. If there had been overlap written into the Nemeth Code, it would not have stood this test of time.
The sample you sent in your first post is a chart or table, not itemized material. Charts and tables follow Literary braille rules (Braille Formats). The sample you show in your second post is itemized (numbered 1.) followed by what you call an unitemized list. It is not clear to me whether the original advice was referring to Nemeth Code formats or Braille formats, as both have "main items with subdivisions." Personally, I do not see "Give some names of small animals, etc." as a list, but rather as directive text. It would help to see the original document.
If the new Braille Formats suggests treating bulleted lists as itemized material, we will have more to think about. If the BANA math/science committee feels clarification is needed in the Nemeth Code regarding this topic, they will write one. I am sure they would appreciate hearing your concerns.