All I'm saying is that their relationship makes favors redundant, be that friendship or work relationship. Remember Mab's offer of canceling the three favors in exchange for his service as a knight? Mab doesn't have to ask Harry for favors, she can order him to do what she wants, and in return she has a specific set of obligations towards him. Same goes for the other two queens.
"Favors" aren't at all redundant; they seem to be something like currency among the Fae. And they are bound by faerie Law.
A faerie who
does a "favor" is
owed a favor in return (the other party is in debt); the faerie who holds a debt is, evidently,
obligated to collect that debt (Molly warns Harry about this, before creating the doppelganger-glamour).
But I think the "three favors" between Harry & Mab aren't exactly replaced by the WK Mantle. The Mantle is wholly a thing of Winter Law, but Mab's "favors" might have been
anything. Now she is limited to the legalities of the Winterfae, the roles of Queen and Knight. There are a tremendous number of nuances and loopholes in those limitations, but Harry is (gradually) finding his way through them. I think it entirely unlikely Mab would have done very much (if anything) NOT in accord with Winter Law, even with a wide-open "favor" to request; but I think she
could have, and that (by virtue of filtering through a mortal) wouldn't have been a violation of Winter Law.