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It doesn't make perfect sense, far from it! Even Rain herself said that there were only 7 BwHoPL during BBS. I'm just one person, but Rain is the last person I'd debate about KH.
Theorizing is fine, but using a theory as evidence doesn't work, then it's what I call, "Wet-cement" evidence.
If it was really AtW's password he would be the last one to forget it and therefore not need to make a hint, but I can think of many reason's why Xehanort or Xemnas would put it there.What reason would there be for him to do this. Of course, the same can be said of AtW, but I think Mickey would've know if something was up.