In "I felt a Funeral, in my Brain," what aspects of a funeral constitute this extended metaphor?

This excellent poem by Emily Dickinson uses the extended
metaphor of a funeral to describe the experience of the speaker which we can perhaps
compare to a complete mental breakdown. At each stage, the separate sections of a
funeral are related to the mental state of the speaker until the shocking finish of the
poem. Thus in the first stanza the "Mourners" treading to and fro and the "Service, like
a Drum" is related to a repeated beating sound going on in the speaker's brain that
makes the speaker's mind "go numb." The box that is lifted "creaks" across the soul of
the speaker. As she imagines the coffin to be placed on top of a plank, that plank
suddenly breaks, and the coffin is left to plummet downwards towards an unspecified
state:

And
then a Plank in Reason, broke,

And I dropped down, and
down--

And hit a World, at every
plunge,

And Finished
knowing--then--

The dash
seems to contradict the words that come before it, because, instead of "finishing
knowing," the poem is left unfinished, the dash suggesting that there is something yet
to come, which might reflect the ambiguous ending of the poem. However, at every stage
we can see that the funeral process is related to the descent of the speaker into some
sort of mental oblivion.