Hm... Well, to use the Asylum theory, she couldn't of been able to kill herself the way she did. Asylums don't have open balconies like that for the patients, for that very reason. So I can see why you don't believe she really killed herself. But at the same time, I personally feel that thinking she didn't kill herself is sort of wishful thinking.
But yes, actually. I don't know anyone with MPD, but with Schizophrenia, it's quite easy to come up with personalities, with places, with complete worlds. I've been doing so since I was a kid.
And actually, it sometimes comes so fluidly that it doesn't even seem like I'm the one who thought of it. Like those people and places were always there, and I just 'discovered' them...
Also, the reason why we didn't see who put the stairs there by the railing? Schizophrenics, myself included, sometimes do things they don't remember. Being able to take notes and refer with other people to basically study myself, this is how it works, at least according to what I've found about myself:
Wake up one day... It's monday. Go through Monday routine. Go to bed... Wake up. It's Monday. The 'Thought Loop' process begins here. Go through Monday routine, go to bed. Wake up. It's Monday. More or less forever thinking it's the same day as it was the day before and the day before that. At least... until something brings to your attention that it's actually Thursday. At that moment, you suddenly forget most of what you did the past three days. Because, in that state of mind, Tuesday and Wednesday never happened.