The word is preferred, or likeliest to be correct. The most parsimonious hypothesis is not necessarily correct, but if a simpler one accounts for all known evidence, it's the preferred one. If, however, some new discovery falsifies that simpler narrative, we modify it to something that accounts for the new evidence.
Right now the theory of evolution is the preferred explanation for the evidence we find today in support of the theory. But suppose something new was discovered that falsifies the theory. If we cannot modify it to account for that falsifying find, we move to the next simplest narrative that does. We would be into deceptive intelligent designer territory now. Something created our world to look like evolution had occurred, but we now know that it hadn't.
Even then, we'll go with the more parsimonious account: a naturalistic one. An extremely technologically advanced extraterrestrial race of aliens who evolved from an abiogenetically created life form were the deceivers, not a supernatural one.
That's a good question that I can't answer other than to say that it's an assumption that has never failed.
Here's a discussion of that if you're interested. The laws of physics WERE different in the earliest universe before symmetry breaking occurred and the particles and forces we know today first appeared, but not since to our knowledge.
If the science works, that's demonstration enough that the assumptions and conclusions underlying it are valid for now. Like I said, new discoveries may require some modification to old answers. Newton's treatment of gravity, though incomplete, was adequate to send man to the moon and back. The Einstein predicted that gravity could bend the path of a massless light beam - something that Newton could not account for. The update to Newton's theory of gravitation was not accepted until this was demonstrated.
Yes. I said as much: "I define knowledge as the collection of demonstrably correct ideas, which only comes from experience (empiricism) or
pure reason (mathematical knowledge, where demonstration is in the form of proof, not sensory experience)." We can know that the sum of the squares of the sides of a right triangle equal the square of the hypotenuse through pure reason. We can also determine that empirically by making such triangles, measuring their sides, and confirming the Pythagorean formula.