First, just the obvious "my simulation doesn't work" does not imply Soul or Astral Flubber exists or that it does something. The more reasonable interpretation is that your simulation sucks.

For the initial simulations yes. If for example humans spent fifty years making more and more detailed simulations and they still didn't work, and there was no sign that they were improving at all, the existence of some really weird new physics shouldn't be discounted at that point.

No, it doesn't. Both could have great impact when combined with a brain, but take away the brain, and you have no function. Harry wanted the continued function of the mind after physical destruction of the body. Even if the mind dependency is Brain AND Astral Flubber, brain gone means mind gone.

This seems like a much stronger point. The evidence shows that Astral Flubber is at least not sufficient, and the brain is really doing almost everything that matters.