Eh... you're mixing apples and oranges. The baseline is that it is super unlikely to happen within a human lifetime. However up until now the recent dimming is exactly the sort of fast event which might precede a supernova. So as of a week or two ago, the Bayesian evidence was that a supernova was about to occur was much, much more likely than the baseline.

It is still more likely to occur in our lifetime than we would have guessed before--these sorts of dust-belching events are likely to happen right before the end. We just can't quantify that likelihood very well because this is the first time we've seen a (pre-)supernova in our stellar neighborhood in modern times.

Best estimates put Betelgeuse at having spent 20,000-140,000 years as a red supergiant, with around 40,000 years being the most likely. We don’t know exactly how many solar masses it is, but under the worst possible conditions (25 solar masses, rotating quickly) we predict that it will persist as a red supergiant for around 300,000 years. For 20 solar masses and rotating, this increases to 500,000 years. Modern models find it most likely to be 15-20 solar masses and non-rotating, in which case it would take even longer to collapse (up to 1,000,000 years from when it began the red supergiant phase).

Even at the oldest it is likely to be, under the best possible conditions for the shortest possible lifespan, it will still likely spend another 150,000 years as a red supergiant before it collapses.

So I don’t believe your statement to be true. We could be wrong about the mass of Betelgeuse. We could be wrong about its age, its chemical composition, its rate of rotation, etc. We could be wrong about our theories of how red supergiants evolve. Any or all of those could lead us to incorrectly conclude that Betelgeuse won’t go supernova in our lifetime. But by our current cosmological understanding, it’s not that “it could go tomorrow or a hundred thousand years from now”. It’s that we’re pretty sure it won’t go for over a hundred thousand years. But if we’re wrong, it’s technically possible that it could happen tomorrow.

What's the relationship between mass, rotational velocity and what we can observe?

A very complicated one: basically the exact motion and temperature structure of the gas of the star greatly influences the spectrum.

So people run spectral synthesis codes of varying complexity, essentially simulating the entire star, and tweak the parameters to get good agreement between observed and simulated spectra.

how can a star possibly be non rotating?

I was wondering the same, but I saw the GP wrote "rotating quickly" in the first use of "rotating", and so maybe "rotating" became a shorthand in the rest of the post.