I've been left with a bitter feeling after reading the end.
So, this whole get up was, in fact, top stop Kosuzu's youkai transformation; as it was, apparently, a matter of time before it happened. So, everything Yukari said in the previous chapter, about protecting Kosuzu from Reimu, was true. Yukari just brought Kosuzu into her plan to protect her without providing the details. So that, by the end, the scroll that would lead her astray and a VERY demoralized Reimu, was used to just counter it.
But, now that she's closer to Reimu and co. and that "she wishes to be able to fight like them", we may very well see Kosuzu as a spell card user in the future. If Marisa, born a simple ability-less human, be able to learn magic and go that far, Kosuzu might as well go that far too.
I still hate Zun's status quo stance.
Also, and dat story gap. Game incoming?
My (inacurrate) 2?.