As you say, in March 2017, BANA approved the Provisional Guidance for Chemistry Notation Using Nemeth in UEB Contexts (available from BANA's website). This document explains that current guidance is to use the opening Nemeth Code indicator and the Nemeth Code terminator to enclose Chemistry material. So, the Chemistry code indicator is NOT used any longer.
The 2015 NBA workshop you mention provided a possibility for the time and is now outdated.
Again, the same code switch indicators are used for both material transcribed according to the Nemeth Code and for material transcribed according to the Nemeth-Code-based Chemistry Code.