It's been a while since I read the early volumes in order, but is the timeline really as close as that? (I can't remember if Yaone ever says specifically how long she's been with Kougaiji.) Kou takes Yaone away from Hyakugan Maoh because the Maoh already has a reputation for raping/killing his kidnapped slavegirls, so it's clearly something he does a lot--it never even occurred to me that there might've been a situation of the Maoh going "Got a nice tasty youkai slavegirl en route to me--darn, she got taken away! Guess I'll go kidnap this human girl named Kanan instead." I definitely think there are obvious parallels between Kou/Yaone and Hakkai/Kanan--with Kou being able to save Yaone from both things (rape and suicide) that Hakkai couldn't save Kanan from, and the connection Hakkai felt for (and the audience feels for him) Kou and Yaone because of that (which gives a really nice and unusual Kou/Hakkai connection that's interesting and surprising because they're dissimilar characters who interact directly very little). Are we really supposed to think that the thematic parallels are actually literal parallels, and Kanan is dead because Yaone is alive because Kanan was switched for her?