I would use …“aren’t we?” if I were part of the performance, and the …“isn’t it?” construction as part of the audience.
The “what a” introduction is at odds with “aren’t we/isn’t it” tag. I’ve never heard or read a sentence constructed with those two disparate elements. I would say that both sentences in the OP are wrong, or at least so non-idiomatic as to make the listener or reader sit up and take notice that something doesn’t seem right. The various rewrites suggested (posts # 3, 15, & 20) all sound fine.
This would be my choice, too. Both of the original sentences sound forced.
This is the most perspicuous question, so I’ll start with it.
The exclamative can be the subject of the clause, but it doesn’t have to be. Compare the sentences (from page 918):
Both are exclamative clauses (they are not subordinate relative clauses). But in the first sentence, “how much” functions as the subject of the clause (referred to as an “exclamative subject”). In the second, “she” is the subject.
So to take your example: “What a story!” Yes, it’s a clause (not in the traditional sense you learned in your middle school grammar class, but still). And it has an understood subject of “it” or “that”. “What nonsense it is!”
To better understand this, think of the case where someone asks “What the fuck is Uncle Charlie eating?” You might answer “my fucking nachos”. But “my fucking nachos” isn’t the subject of that answer - the subject is an understood “Uncle Charlie”. But you don’t need to say it.
I’m now treading closer to Left Hand of Dorkness’s post, so I’ll pick up there.
This point is well taken. I refused to accept it the first time, but perhaps, maybe, possibly, the subject of the sentence is an understood “it” and the rest is a relative clause, which would, indeed, make “isn’t it?” an acceptable tag at the end. And this would mean, exactly as Dorkness indicated, that both choices could be correct. And in fact, I’d say that the vast majority of these GQ grammar questions actually end up with both choices as acceptable. I should’ve been more careful when my initial analysis concluded that one was better. It’s so seldom the case.
But the first option is also correct. Just because we can look at “we’re having tonight” as a contact clause (a relative clause with a missing relative pronoun) doesn’t mean that we should force this interpretation onto the sentence to the exclusion of other exclamative clause constructions.
“We” could absolutely be the subject of that clause. And so we are once again left with a situation in which the questioner is free to use their best judgment in determining how to proceed.
And I’d say the best choice is to avoid this construction. Both are grammatical, but both are also weird and stilted and best avoided.