I have an answer to my first question:
I had assumed, since “tokenTable” is created and added to the simpleMode object by hand, that it was a convenient storage place for custom Tag objects. However, experimentation has shown that it is not only necessary, but MUST be in place before the simpleMode object is passed into StreamLanguage.define(), or the highlighting doesn’t work.
I retroactively found this thread, where CodeMirror was patched to add support the optional “tokenTable” for “streamParser” objects. It looks like the “simpleMode” method generates a kind of “streamParser” object, so this all fits.