Regarding 3, there's no way to find evidence against it (or for it, for that matter). You can't look at a given system and measure its sentience. The closest to that anyone's ever attempted is to try and test intelligence, but that assumes cognitivism/computationalism, among other things.
I agree with orthonormal, except that I don't have $10,000 to bet.
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.