This is my theory.
I think it's the only explanation about Sarada that can help Sakura's character from being degradated even more (even if it's implicit that Sasusaku never had a kiss). And Karin's, of course! Why would a mother leave her child in the care of another like that?
Sasuke's DNA in Karin's drawer theory is just ridiculous. Well, the whole story of Naruto Gaiden is pretty ridiculous (including that kitten villain) but as the king of the obvious, Kishimoto can't be any more ridiculous like that. Or can he?