I can't even imagine how hard this hit you and your family, Julie. Such a shock that he was gone so suddenly, but worse are the circumstances, and right before an occasion that is notorious for being an emotional day anyway - Christmas - so it's going to be with you every year on that date.
I'm so sorry you haven't been able to find peace or justice for this, or have any of your questions answered.
I pray that you and your family find whatever it is you need to be able to come to grips with this - best you can - so you can move forward.
If it helps, I've been communicating with spirits since I was a kid - not my idea, but they insisted... I can promise you, your father is always near and he hears you when you speak to him or think of him. Our loved ones always stay close in spirit, no matter what.
Wishing you whatever you need to be able to heal as much as possible in these tragic circumstances 💜