From what I'm assuming from the profile information, she and Marisa already know each other in some passing fashion, maybe.
Also, Ammy, I wouldn't say these new developments really retcon anything in particular, it's just that we've only really known Reimu's family and Yukari were part ofgensokyo's creation; which would mean Okina has ANOTHER "hidden" thing about her and her involvement is only being brought up now. Kind of conjecture, though, honestly, and unlike some people *coughsummerscough* I will freely admit it is only that.
cuc: That IS a drum, actually. One of those small, flat, handheld ones. it just looks like a mirror because of the perspective, and I've already seen fanartists draw it as such.