What I dont get about the Yachiru is Ken's zan theory, is that Zangetsu actually heard the actual spirit within Kenpachi's Zan screaming. If Yachiru is Kenpachi's Zan spirit, then what did Zangetsu hear?
it could be something completely contradictory to him like a K type or it could be a defensive type to fill in the gap in his power since his main thing is offense
Kenpachi said his reiatsu gets sharper when he is excited allowing him to cut things.
One's ability to cut with a Zanpakuto is based on how well one can focus thier reiatsu.
A person with less reiatsu can actually break thier zanpakuto on another shinigami with a higher reiatsu. This is why Ichigo couldn't initially cut Kenpachi and when Ichigo relaxed, because he thought Kenpachi was like him Zangetsu got cut in half by Kenpachi.
Yachiru was given her name by Kenpachi, when he first met her she did not know her own name and so he named her and himself.
How ever we have also seen Zangetsu wield himself and variations of himself when Ichigo was trying to reach Bankai.
So its entirely possible that Yachiru is wielding herself complete with full Bankai.....