Hmm. I always rationalized that the scene where Nago snatched back IXA and attempted to play a prank, was to let us know how pathetic he really is without IXA.
Remember how cocky he was back when he was IXA? and how he was always being arrogant and an A**hole?
I feel that it just shows he's nothing without IXA, his confidence, etc... relied too much on it. If anyone noticed, he kept powering up whenever he could, (powered IXA?). Plus, there were the many scenes where Shima accosted Nago about him being weak, which we finally see here.
As for the Wataru arc, i felt it was similarly out of place, would have expected him to at least fight Wataru and we would have gotten a bada** IXA-Kiva fight.