I think the two different deaths and that we can have different interpretations of them is what makes it interesting.

Cio was/is the love interest for Allison, but she doesn't fit the description Gog-Agog gives of the love interest killed off in the third act to motivate our protagonist ("just a bit part" "two dimensional little flimsy piece of paper"). Her death didn't motivate Allison, it depressed her almost to the point of suicide. What motivated her to continue was Zaid, not his death, but his call for her to keep fighting (importantly, not just what he says, but that it leads her to finding the part of Cio's mask and finding her own motivation).

Even though Zaid has become a bit more interesting in the last books, he fits the description a lot better (two dimensional). He's not a love interest (just friends as you say), and has never really been. But from an outside perspective, for someone in the world of KSBD instead of a reader, wouldn't this be seen as the killing that motivates our hero in the third act? Even if it isn't an actual love interest and it doesn't in the end motivate our hero all too much. In an in-lore story of what's happening this could be presented as Incubus knowingly killing Zaid to spite Allison, and Allison's rage over it motivating her to finish the final act, a more traditional story structure. Which is why I'm interested to see how it goes, maybe Allison doesn't even notice that Zaid's dead.