There is a general misunderstanding regarding the meaning of words like "law, principle, theory" and such within mainstream culture. In science, everything is a theory. Scientific knowledge is always probabilistic. A currently accepted theory is only accepted because it is confirmed by most observations, make for better predictions, and/or provides the best answer to a previously unanswered question. When scientists use the term law, they simply mean an established principle without any known contradicting evidence (or very little). But it is still probabilistic. The constancy of the speed of light is a law of physics. It has had so many confirmation across so many different fields and disciplines, and it explains so much in a profound and comprehensive way, that it would require some SERIOUSLY convincing experimental data to cast doubt on it. But it may be proven incorrect nonetheless. An accepted theory will typically contain many laws and principles.
The multiverse is a theory in a softer sense. Whether someone believes that interpretation or any other makes little difference to their experimental procedures, as the interpretation has little bearing on the rules of quantum mechanics. QM is the theory. Multiverse is the explanation of why QM are so freaking bizarre and counter-intuitive.
