[William Cloud Hicklin]

IIRC, in that passage Turin was said "to be numbered with the sons of the Valar." This is problematical in a couple of ways, starting with the fact that Tolkien definitively rejected "children of the Valar" and converted them to Maiar; but also that "numbered among" isn't quite the same as "is."

It's obviously impossible to fit Turin slaying Ancalagon the Black into the narrative.
Surely not *impossible*; one could suppose for instance that Turin was in Vingelot with Earendil, possibly even doing the fighting while Earendil drove the boat in a sort of Trojan War charioteer/spearman arrangement. Perhaps a not unfitting pairing, given that both fellows had non-standard relationships to the Doom of Men.