Or maybe she just didn't realize she could do it. She doesn't have to be all-aware like you suggest she will be. Hell, if they turned her into even MORE of an all-achieving Deus Ex Machina, pulling some "I'll stay here forever because I don't want no Darkness stains on me" I'll consider her character more shitted on. I don't mind her being weak so long as she eventually grows out of it, and she can do that without denying the Darkness actively or dooming herself for exile.
Xemnas being a Nobody is actually quite possibly what let him do it - he lacked a Heart. KH filled that void in him. Aqua wouldn't have that. Ven and Sora managed to bind their Hearts because Ven's Heart was fractured, too. Aqua and the RoD KH are - as far as we know - whole.
Even XH and MX managed to possess Riku and Terra once they became existences of only Hearts. This does mean that KH might 'possess' Aqua but going by past examples, more so than Aqua wielding KH, it'll be the other way around. Unless they Deus Ex Machina her to be able to control that thing which is again, imo, a fate much worse than her needing Sora to save her.