When I was expecting (and hoping) for a concrete answer to the ambiguity of the vampire-sucking-the-blood-of-their-beloved matter that's now cleanly separated Kou and Nazuna for a year, my mindset was that it had to be answered because it was an ambiguity-- more so, a conspicuous ambiguity whose unclearness was drawn to attention multiple times. However, looking back at Hatsuka expressing how vampires hardly understand themselves (ch. 44, p.11-12), I think I may have been misguided.
The mechanics behind any of the abilities and circumstances of vampires are never explored extensively because Kotoyama made it a point to only explore them through the characters; not only are the characters only able to attest to what they experience and understand, they don't have much means to explore for an answer. Hatsuka doesn't know why they the blood of failed candidates tastes bad to them (similarly, there's a lack of clarity of the meaning of Nazuna finding Kou's blood "especially tasty", especially since he's not the first person she's fed from). Nobody can explain why treasured objects from one's human life can immobilize and kill vampires; in fact, they only seem to become definitively aware of it because of Kyouko, but even Kyouko-- who's extensively explored the nature of vampires through her hunting of them-- wasn't aware that they could become intangible (cf. ch. 55), so it's not like she knows a lot. Nazuna knows how to phase through things, but needs Susuki (a fighter) to explain how said intangibility can be strategically used (ch. 128, p. 8-12). Especially, neither Susuki or Asami can attest to why vampires are so averse to sucking the blood of a subject they love, or why vampires are prone to interacting with humans as if they're playing a game of seduction-- they can only attest to it being so counter to a vampire's instinct, that there's a rumor that sucking the blood of a beloved subject would kill the human (ch. 121, p.14-15).
They have zero idea about the nature of Kou's vampire shifting beyond Kyouko's admittedly reliable conjecture-- nothing like it has happened in their present memory. If it happened because Nazuna is a dhamphir, or because Kou himself is unusual, they can't test that and there's nothing to compare their case to. More pertinently, they have no means to safely test whether the aforementioned rumor is true in any capacity. All they know is that they (normally) have an instinctive aversion to sucking the blood of humans they love, and the last vampire to do that (assuming she truly loved Mahiru) seemingly died because of it. Because of those two factors, and because Kou and Nazuna truly care about each other and actually just want to be with each other at this point, there's no value in even risking one of them dying.
In short: there isn't a narrative device that the author has shown herself willing to use that's also consistent with the characters and their MOs, that would answer any of the remaining ambiguities. Meanwhile, Kou's definitively conquered the malaise within him that started this narrative, and everybody else's character arcs (if they had any) are complete. Nazuna is almost certainly showing up in the final chapter, and what becomes of the two of them will be revealed.
Upon a partial re-read, I think what we're getting is by far the best possible outcome-- not even remotely downer. Particularly: even before Kabura revealed that Nazuna was born a vampire, the prospect that Nazuna fell in love with someone that she forgot when she was turned was something that really bugged Kou (ch.56, p.11). Even if neither of them died, if Kou became a vampire, it would have caused him to forget the treasured times he had with Nazuna-- on top of thoroughly effacing his character development. I think that was part of the value of the montage of memories at the end of chapter 197-- they're making sure to meet again, and Kou gets to keep everything he gained as well as already had.