There were 4 water tanks. Elisa was found in one of them.

Yes, she would have been able to open the lid. None of the lids were able to be locked at the time of her death. The tank she entered had a completely flat surface to stand upon while removing the tank. This is California, so you wouldn't have to worry about the lids being "frozen." It would have maybe taken a tiny bit of elbow grease, at best, but she would have definitely been able.

Yes, they found water in her lungs. She definitely was alive when she entered the tank because the coroner's report said she died of something like "rapid/shock drowning."

Another question you would have to consider: how could someone fit an entirely limp body in that very small square opening without leaving any flesh, blood around the brim or any scrape marks on the body? It would have been impossible. The police had to cut a hole in the tank to remove the body because they couldn't remove it from the opening where she had entered; so it's safe to assume that inserting a limp body into that same opening would have been just as difficult (unless she was somehow bundled into a straight package, then dropped in, but she wasn't -- there was zero indication or evidence of such found inside the tank).

There were zero killers. Elisa didn't necessarily commit suicide. She suffered from a bi-polar episode. In the elevator video, she's being playful with herself and doesn't understand why the elevator won't move, so she enters and exits and moves her hand to try to trip the censor, etc., before giving up and casually wandering out of the elevator towards the stairs.