In the German and English versions of the same game I ran across:
Er hätte heimkommen sollen, ist er aber nicht.
He should have come home, but he didn't.
(This is said by a woman whose brother has gone missing, and she's very concerned about it.)
I'm having a hard time understanding the word order in the second clause. It seems clear that a redundent heimgekommen has been dropped, just as a second "come home" was dropped in the English version. But I would have said ..., aber er ist nicht (heimgekommen) with the verb in V2 position where it belongs. But not only is the verb in V1 position, but the conjunction aber seems to have been turned into an adverb and moved to the middle of the clause. So I'm confused about why the word order is acceptable.
My current theory for the V1 issue is that the missing heimgekommen is in the first position. So the complete second clause is ..., heimgekommen ist er aber nicht. For the issue with aber, my explanation is that it has been turned into a modal adverb (also known as Abtönungspartikel). Marcia's answer here seems relevant for this. A conjunction joining the clauses isn't really needed so none is used. Are these explanations plausible, and if not then what is the explanation?