Correct me if I'm wrong, but theologically only the nephilim concept exists, with various ways to write it across the ancient texts mentioning it, while nephalem is a modern fiction expy of it, mainly found in the game Diablo. So, Nakaba wouldn't be in the wrong to call Tristan "nephilim", whichever combo of races he is, especially since in some texts, the punishment for the Watchers, the group of angels who fathered them, and for a portion of these very nephilim, was to be turned into the demons of our current perception, who would tempt the humans into sin. Moreover, what with the various translations for the terms describing the divine fathers these "giants" had, sometimes the rebellious and sinful angels are addressed as fallen angels, again, later perceived as demons directly...and what with the portrayal and origin story Nakaba gave the 2 clans, demons and goddesses, they're all the "children of god(s)" and their males would be the biblical "sons of god" some way or another. So, whichever way you slice it, Tristan is nephilim, even if he's a combo of all 3 races.
BTW, I forgot to mention it, but I think it's not the Q&A that is relevant towards Tristan's biological make up, but rather what Mel himself said during the discussion between the group, Thetis and Mel on what Arthur is doing to Britannia's landscape and Camelot's barrier. Unless the translation is somehow wrong there, he specifically said that Lance and Tristan are "part non-human"(referencing Tristan's goddess and demon legacy as "non-human" and making the other "part" as "human").
