Having read the entire topic again, my opinions start to shift towards the original post more and more. 
I just don’t see the need for the three dashes. Thinking of the writers I have to instruct on how to type their reviews, I think I will tell them this way because so far as I can see now it has the balance between being easy to read and easy to type.
The gig was great!
! {.setlist}
1. Track One
2. A Different Tune
The after-party was OK.
The proposed variations in character (both exclamation mark and semicolon) and fencing (delimited and side-marked) are fine. The proposed syntax for nesting is also great. Not sure whether the end delimiter can be a blank line - I would like to have that explicitly mentioned in the definition when this goes live.
I am assuming the {.className} notation for classes and attributes that has become to most-used syntax will make it into the CommonMark definition one day.
The least controversial way forward, I agree. Just not on the three dashes. 
Well, I do see how it makes it better readable, I won’t vote against the three dashes.