Snipped for context.
I think you make some really good points here. Just some points on it. If she was coerced, then really at the end of the day we are speaking about a form of kidnapping (until the death). What would cause Elisa to be coerced? Likely a weapon in my opinion, but it is very possible to do without if there is physical intimidation, so forth. I would argue that if she was coerced, at the moment right before entry, she would likely have known she was going to die (assuming mental state is intact, so forth). Should have been a struggle given my experiences. However, again, not always the case.
At the end of the day, what my experience is asking is why the tank? Were I to do it, I wouldn't put someone alive in a tank. My victim would scream, cry, beg, struggle. All things I can't deal with in an adrenaline state. You are going to want to subdue or incapacitate your victim in some way, but then that's not coercion. You put her in unconscious in a water tank - well that's murder.
While the mental/drug area is not my expertise, entering the tank accidentally/alone willingly can only mean one of the two, no? It is unlikely a rational and logical person would enter the tank, unless of course it was a dare. But then how would she have died? A dare turned tragic? Persons involved too scared to come forward and left the body? Plausible.
Edit: Also the whole naked thing. I'm not sure where this fits in.