Psychotic or no, drugged or no, Elisa was shown on the video trying to go down. Whether or not the roof was accessible by "scary" ladders, I really don't think she suddenly changed her mind and decided to go up.
If I were her family, I would be livid right now at the notion that her death might be ruled misadventure. Everyone who knows her and has spoken of her online paints the picture of a larger-than-life but SOBER personality.
Flip-flops (the footwear she is shown wearing in the elevator) are not great climbing OR swimming footwear.
If you decided to climb, you'd take them off and change shoes, or you'd abandon them by the beginning of the ladder. Maybe they'd fall during your climb (all the way down to the street, or onto a ledge of the building).
However, if someone is carrying you up, the flip-flops are also likely to fall off: and the person carrying you is going to want to cover his tracks and make sure the flip-flops aren't lying around where they fell.
Assuming you got to the top of the roof with your flip-flops intact, you wouldn't jump into the tank IN your flip-flops, would you? No way. Have you ever stepped into a pool while wearing flip-flops? They become serious impediments. If you got to the tank on your own steam, and for some reason wanted to go in (some lunatic idea of making it into your own private pool), you'd kick the flip-flops off, leave them at the side of the tank. Of course, if you got stuck in the tank, the police would find the flip-flops later -- and I assume, since they searched the roof already, they didn't.
Again, though, this is assuming you were alone on the roof -- nobody there to remove the flip-flops as they departed, and left you behind.
In other words, I'm curious: did she have other shoes? And where are the flip-flops we see her wearing in the video?