One question I have is that if two people use the same prompt, do they get the same result?

The process is deterministic, but does not rely only on the prompt (or other forms of conditioning), the model itself has to be the same, then the PRNG seed, and, to varying degrees, configuration parameters such as step size, image resolution and configuration (meaning, roughly, "how much should the process weight your prompt over the model".

The more detailed the prompt is, the less creativity the process will show, and at least in my book the more can be attributed to the human. What can also be attributed to the human is sifting through multiple seeds with a single prompt until you find one that's just right. Coming up with various process pipelines. Creating input for the model, such as depth maps.

Generally speaking you shouldn't liken the human input to the process to painting, but to art direction. If I tell a painter "draw me a tiger in a forest" then no court ever will give me copyright to the image, if I write a whole novella to describe the image, influence the artists' output sufficiently, I'd get acknowledged as a co-creator. AIs not being able to hold copyright themselves, once you hit that co-creator threshold you should have sole copyright over the work.

...and that's nothing new that's just how copyright works. In the Anglosphere you have the sweat of the brow doctrine and coming up with the sentence "yo draw me a hot chick with big tiddies" does not produce sufficient amounts of sweat, in the continental tradition it's threshold of originality, and no that instruction was not sufficiently original.