Yes they were neglient and if it wasn't them who killed her (assuming she is dead) I am pretty much sure the guilt they feel far outweighs anything anyone can ever say.
I'm hugely suspicious of the parents. They were the last reported people to see her alive, no one came forward with clear information to collect a substantial reward, the alleged blood in the room, the alleged blood in the car. Not a sniff of a sighting. You would think it is up to them to prove their innocence. To be fair, they are lucky they haven't done time for this.
Don't remember there being blood in the apartment or car. Maybe I am forgetting. There was 'DNA' found in the car but it was dismissed/explained I think? I thought the accusation they had faced in terms of them killing her was that they had sedated her being doctors and it had gone wrong, so why would there be blood? The thing that makes me think it's not the parents is that they are continually pushing for the investigation to continue. If they did accidentally kill her I think that they would have let it quietly come to an end long before now. That bit doesn't really add up for me and as for the "they look guilty" theory, well, case closed, rock solid proof there, lock them up and throw away the key. No arguments from me that they were negligent leaving the kids alone, though.
