Hypothesis 2: Schroedinger's cat is simultaneously both alive and dead. Our taking a look will "collapse the wave function" which could then result in the cat dying.

Is there any concrete evidence to believe in hypothesis 2 over hypothesis 1? If not, I'd rather go with the hypothesis that doesn't sound absurd.

Say we flip a coin (just for the sake of the argument - superposition only works for small things), and we're arguing whether the coin is really heads or really tails before we look at it, or whether it's in a superposition. The bell inequalities give us a prediction on the distribution of results we should see if 1. was true or if 2. was true. Turns out the distributions we see in experiments are consistent with 2 but not with 1. The measured values can not be true if the coin is either head or tails.

Of course, there are subtleties involved, but that's one of the main evidences we have.

https://en.wikipedia.org/wiki/Double-slit_experiment. That's pretty concrete.

Plus all those fancy quantum computers that people are developing depend on the fact that quantum superposition exists.

But you can't translate the quantum stuff to the cat, because the detector makes a measurement. So because of the detector, the uncertainty is gone, and there's no uncertainty in the state of the radioactive atoms, let alone in the cat.

Almost the entire idea of quantum mechanics is that hypothesis 1 is incorrect.

Even for photons sent through one at a time? Nope. That's not behavior associated with any type of wave.

It works when you send electrons one at a time, suggesting they are somehow interfering with themselves. In other words, the electrons are in both slits at once until we check. That might not literally be true, but it's not possible to explain the interference by saying we just don't know which slit they're going through.