Fortunately we have troves of her handwritten documents; all of her poems were first printed posthumously. To me, she's using the punctuation as pacing or tonal markers as opposed to ligatures ("I'll clutch— and clutch— " vs "I'll clutch-and clutch-"). Many publishers style these marks as longer than normal m-dashes for that reason, which makes sense seeing as they are rarely used as asides.
I interpret her marks—
as breathless pauses—
that— having no unicode—
should be given to m—
and space—