"Now that he's gone, we should make our own life."
If "now that" and "because" are interchangeable, it would be okay to change the phrase "now that" into the word "because".
1.Am I thinking properly?
2.And is there any difference in nuance or shade of meaning between two expressions?
No. "Now that" refers to a transition.
He gone. As we move to the stage in life where he is no longer with us, we should...
It doesn't have to be so dramatic.
Now that we have taped the trim, we can begin painting the walls.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.