I have stayed out of this thread until now, but I'm going to say this.
You are not looking for an unbiased opinion. You've got six pages of unbiased opinions, all of whom say that if there were sufficient evidence to support your theory, you would not have to be making these arguments because if there were sufficient evidence, the police WOULD investigate further. The fact that they are not, shows that there is NOT sufficient evidence to support a claim of murder.
The fact that six pages later you are still telling all the unbiased opinions that they are wrong, says that you don't want unbiased opinions, you want someone to agree with you.
I think we all understand that you are grieving and upset and want what you perceive to be justice - someone has to be punished for your father's death. No one blames you for that. But there is going to have to come a time when you accept that if there were enough evidence to charge someone, someone would be charged. No one in law enforcement, not the police and not the DA, wants to let serious crimes go unpunished. But they can't make bricks out of straw, and they don't have either the time or the money to be putting all their resources into your father's case when there isn't sufficient evidence to charge someone or enough to work with to create new straws to make bricks with. There are other crimes, and other fathers, and other families. They deserve a share of the police's time, too - it can't all go to you and yours.
You have my sincere sympathies on your loss. But, and I say this out of kindness, not rudeness; it's time for you to let go.