I tend now to agree with your assessment.
The Guidance for Transcription Using the Nemeth Code within UEB Contexts helps to clarify this point in item #5 under Additional Guidelines.
5. Measurement units (e.g., feet, ft., min) adjacent to related numbers transcribed in Nemeth Code are part of the technical expression and are transcribed within the Nemeth switch indicators."
Now that we have the Guidance to study (This document was not available when Trumbull and I started this discussion.), I think we can conclude that measurement units which should be included in Nemeth mode are those which are standard units of measure, namely from the US customary units system, the metric system, and the International System of Units. Since, "bag" is not such a "measurement unit," it should not be transcribed within Nemeth switch indicators.
Of course, Trumbull provided the best, most consistent transcription he could given the information available at the time. With the full Guidance now available, I think we would transcribe “1/2 of each bag of popcorn” with only 1/2 in Nemeth and “3/2 bags of popcorn” with only 3/2 in Nemeth.
How does that sound?