«…when I try to communicate the subtle difference, when writing an appositive phrase, between using commas, parentheses, or (the connoisseur’s choice) the em dash.»
I HATE the em dash – along with my favorite, the parentheses (they’re so overused, and poorly).
Draw a period and write a new sentence. If you can’t link it to the previous sentence gracefully without disrupting the flow of what you’re writing… maybe it doesn’t belong. What are you trying to do?
P.S. This is actually a pretty good essay.