You know I think I explained it already in a way that you should be able to understand if you wanted to.
I'll try again.
Your weighing of the coriolis force against the acceleration through gravity caused by a slant is not representative for the problem of the entire sink. It is only representative for a tiny small area in the sink, which is on the surface.The coriolis force acts everywhere where the water is moving, in the whole volume. And even looking only at the surface, you would have to integrate over the whole surface and prove that the effects of the slants or irregularities in the sink have an effect that creates rotation in the integral. The gravity/slant effects may just as well cancel themselves completely in the integral.
To correctly prove if its negligible or not, you would have to integrate over the whole sink. Which depends on the shape, size, water viscosity, time it takes to drain it completely. The larger the sink and the more time it takes to drain it, the larger the effect of the coriolis force will be. That is not reflected in your calculation at all.
You are correct in saying that the coriolis force is negligible in the average sink. But your calculation alone did not prove it.
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