This act of preventing a future event is known as "bilking" and is a pretty sound argument against time travel. However, bilking is impossible for entangled particles.
I'm talking about backwards causation as a general principle.
On macroscopic scales not much changes since backward causes are limited...
Says who? What is the definitive study of backwards causation? I'd like to see some sources which claim that violating causality would not cause experimental problems. What about simple particle physics experiments where we are working on microscopic scales?
Moreover, sometimes science and mathematical calculations are hard. But that's the way the world is and the simplicity of calculations can't stand against the reality of observations. Calculation difficulties have been around since the three body problem.
You're not understanding my point. I didn't say the calculations or experiments would be difficult. I said that in any experiment where future events would have to be taken into account, you couldn't make definitive statements about your results. If I do an experiment to show A causes B and future events can also cause B, there is no way for me to state definitively that a seemingly positive result is caused by A and not some future event I can't control for. This is what makes causality so essential for science.