I'm adding another answer because IMO it's different enough to merit a seperate answer. But I know think it has to do with:
And based on your responce to my last question, I'm going to make another stab at it with a
Forward slash. Which is either a line break, or a period/comma.
Mutter dos de thou around a rhetorical shape,
Reference to a couplet (dos) of poetry. Slashes are also a rather simple shape.
then pause with the hero of Card's game.
Ender is the hero of Orson Scott Card's book Ender's Game. A line break ends a line and so could hypothetically be refered to as an ender. A forward slash was also once used to denote either a period or a comma which end sentences and clauses respectively.
Then let it sit for far too long
I think this is a reference to a caesura which are often denoted
while the celts tell you how many bards came.
Possibly a refence to Shakespeare?
Our protagonist makes another appearance.
This sentence exists for poetic adherence.
A short commercial makes it done and unique.
Again, I'm unsure.
If compelled, you can choose to mark it oblique.
Oblique is another name for a forward slash, and this makes me doubt my answer the most and think that perhaps it is not a forward slash, but rather a line break in general.
I have no idea if this is closer or further from the true answer, but even if it's not perhaps someone else will get closer from my answer.
What I know so far based on @puzzle-guesser's replies to my answers:
The answer may be about punctuation but not definitely. It is something that ends somthing (an ender), be it a comma or a clause or something else entirely. The last line for sure refers to a forward slash.