At the moment, as we could see many times, he is clearly enjoying to troll the readers, so I wouldn't keep my hopes up on Karin being the legit mother. He might reverse troll us on this one.
Kishitroll saw how successful (in terms of $$$) his shojo Naruto movie was and realized that there's merit ($$$) in shojo'ing Gaiden up with Maury-esque baby-momma dramas, dna results and pairings. There'll be lots more to come.