"The fight cost him his life, almost" is grammatical, but placing "almost" at the end is just bad writing style. It detracts from the drama of the sentence by making it sound like the writer isn't sure what to say. More natural would be:
The fight almost cost him his life.
which negates any problems with the comma.
The second sentence is incorrect from the start by misusing "lay" (Lay vs. Lie). You lie down on the grass. You lay something else down on the grass. In this example, she should have been lying in the grass, looking up at the stars.
Otherwise this sentence is an example of creative writing, where the usual grammar rules can be broken for artistic effect. Take this excerpt from "The Road" by Cormac McCarthy:
When he woke in the woods in the dark and the cold of the night he'd reach out to touch the child sleeping beside him. Nights dark beyond darkness and the days more gray each one than what had gone before. Like the onset of some cold glaucoma dimming away the world. His hand rose and fell softly with each precious breath. He pushed away the plastic tarpaulin and raised himself in the stinking robes and blankets and looked toward the east for any light but there was none.
Notice the lack of the usual punctuation, and numerous sentence fragments -- unacceptable in an essay or a newspaper article, but perfectly fine in a work of fiction, or in poetry.
As you guessed, the full stop in the third example is incorrect because it creates a sentence fragment. It may have just been a typo or a bad edit. However you don't need a colon after "commas", as a simple comma would do. Personally, I would change the punctuation:
In the second sentence because "John" is set off with commas, it is not necessary. It is added information.
