Yes, but that doesn't mean (s)he's a pure-blooded taguel, nor does it mean (s)he's just appearing like Morgan is supposed to. We don't know the effects of being only half-taguel or half-manakete. It could affect absolutely nothing with regards to taguel transformation. So really, I don't even understand the argument you're trying to make here.
Look, I'm all for having headcanons, but not all headcanons are good headcanons. "Yarne (and Morgan) are adopted" are not, in my honest opinion, good headcanons. It doesn't really make a whole lot of sense. If there were another group of taguel that they could have come from living somewhere on the continent, well, Panne would probably already know about them... and so would the regular humans who killed Panne's brethren. Honestly, those taguels would have been killed or discovered by someone else before Panne could have found them.
I guess I should provide another example, yeah? Here's one possible headcanon:
Well... that doesn't really make a whole lot of sense. It's kinda silly, don't you think? By contrast, here's another possible headcanon:
See, isn't that a lot more logical? Dark Magic is, after all, known to have some pretty serious effects on its user. The skin thing could just be another one of those side-effects.
Now, what would make more sense as a headcanon than yours would be examples of what their half-human, half-taguel heritage means for them. Does it affect their strength in their taguel form? Does it affect how long they can transform for? I don't know, but it makes more sense than them being adopted.