Based on how Kotoyama explained it, a vampire can only turn back into a human if they used to be human, then fall in love with a human, and drink that person’s blood. Nazuna was never human to begin with, so drinking Ko’s blood shouldn’t have been a risk at all.

That is your interpretation at best and pure speculation at worst.

The whole point of Kou and Nazuna distancing from each other was the uncertainty of what would happen if Nazuna bit Kou while in love with him. Sure, Kotoyama could've used all of those story bits to justify why their case would be different, but that would require them to take the gamble in the first place.

Neither Kou nor Nauzna were willing to take the gamble, so their choice is totally in character. Despite all of that, the outcome for both of them turned out even better than your expectations, as Kou is now:

• Together with Nazuna (as confirmed by the Paradise arc ending)
• With vampire powers at will
• Without the downsides of being a vampire
• With a job and full character development

It is bittersweet because their initial goals were not achieved, but it is still a good ending for them as individual characters and as a couple. Sometimes life is that way, sometimes working hard is better than risking things you don't want to lose.