The big difference is that the atheist arguments are not reliant upon Muslim philosophy in the way in which all possible MWI arguments rely on physics. The relevant questions are subtle aspects of physics - linearity, quantum gravity, etc. related, there are unresolved issues in physics to which many potential solutions are MWI unfriendly - precisely in the ballpark of your "quantum gravity vs. string theory" where you do not even know how two terms relate to each other, nor know how advanced physics relates to MWI.