I had most of my response typed up, but then my internet went and froze so here's a sloppy, condensed form.
Sure, Debbie had dark hair, and so did Ethan, but then so did Martha and Aaron. Lucy and Ben were the only ones with lighter hair.
Did Debbie really seem like a favorite? Ethan may have been tender with her, especially in the whole scene about the gold locket, but at first he mistook her for Lucy, who was considerably older than Debbie, after all.
I don't feel like it's based on pure love for family that motivates Ethan on his quest. It's love for Martha, sure, and a desire to either stop miscegenation before it happens or to solve it by killing her, and also just that he has a psychopathic and obsessive personality. Your point is obviously well-taken that the Searchers is a deeply complex work, as are so many of Ford's, and that it wouldn't be entirely out of the question for Ethan and Martha to have actually had an affair in the past.
But with that said, I truly do feel that Ford's entire stance, critical though it may be, is still very Catholic, not so much in its morals, although that too is the case, but in his sense of the irrevocable loss that so often occurs when various ideals come into conflict. Consider Hallie's love for Tom despite being married to Ranse in <I>Liberty Valance</I>, or the devotion to their dead wives or lovers (Abe Lincoln, Nathan Brittles, Frank Skeffington, &c.) that so many of Ford's heroes demonstrate. Ethan's love for Martha is, like so many things about him, a perversion of one of the qualities of Ford's true heroes, but is also tragic in that it was likely never consummated.
Sure is nice to have someone to have a good conversation with about this film, though.
Oh, and an aside to PhantomStranger - I really don't think "inferior" sounds like any kind of value judgment other than a relative one. It's a lesser work than the other two in the trilogy, and there's no escaping that, but nobody's saying that it's bad, just that it's inferior. However, I do disagree with you on which is ultimate superior - that in Fort Apache, Henry Fonda's Thursday, and the heartbreaking final scene in which John Wayne's York defends and becomes him eclipse both the Pony-that-Walks scene and the "Lest we Forget" watch scene in She Wore a Yellow Ribbon.