OK, I guess I was ranking the conversation between Barty and Dolohov as being more reliable than the exposition from the ISS to Lupin, since they weren't actually able to see what was happening and were probably in a bit of pain/panic that might have clouded their perception.
I agree that it makes more sense for Dolohov to have killed her (and ties nicely back to when he told Seamus, after the Dark rite, that if someone has to die, then better that they die quickly than in agony.)

That said, if Dolohov had promised Selwyn that his family, particularly his children, would not suffer unduly for his mistake, then maybe that was what he made good on by letting the ISS go free? Not causing harm to innocent magical children could be a matter of scruples.

I also found it a nice little touch that when he writes his last message to Selwyn, he describes Selwyn's choice only as "ill-advised". No moral outrage for Dolohov!

Finally, yeah, I am looking forward to finding out where they went--perhaps to find out who brought the Selwyns in?--and what Barty did on his side investigation when they came back for Chloe's body.
(And call me crazy, but the fact that Dolohov says "we" when talking about getting Chloe's body back gives me hope that Barty supported the idea of not leaving her there for the DoM, even if only out of spite.)