Why try to drive the daughter crazy if they were going to kill her anyway? I could understand it if they had planned to make it look like a suicide, but it was set up to look like an accident. Maybe they changed their minds because she wasn't going crazy? If they had changed their minds, it should have been explained in the script.
Apparently they did want to "drive her crazy" first, but why then was it always the driver, one of the plotters, to promptly point out things to prove she isn´t losing her mind after all? And, furthermore, how was he able to maneuver the old dude to the front seat unseen the way he seems to have done (not to mention, it would have been easy to establish he´d been dead for days, so the whole idea was kinda silly to start with). Also, wouldn´t the chick have been somewhat banged up by the fall, to say the least? Not to mention the whole plan of her becoming the impostor for the drowned cripple doesn´t really make much sense... but, without it, of course, there would have been no film. In all, well shot movie and quite gripping while it did last, but these kinds of things do drag it down somewhat.
Well it depends on how hard they actually tried to drive her crazy. They knew that Penny was a very emotionally and mentally unstable person, they probably figured just seeing her dead daddy would be enough to drive her over the edge.