I also had a near death car accident and what you describe is virtually identical to my own. The same thing happened, instead of fear a calmness came over me and I thought - its my turn to go.

Your experience at the Grand Canyon is also similar to something similar when I was climbing a mountain with a friend. I was half way up and looked up and down. I suddenly felt very small in the universe but at the same time I felt part of everything.

Regarding Niagara, I have heard about this force that has a pulling affect on people making them want to be part of the energy. That alone tells me there is something more to our existence.
Great examples @bobreedo