These issues will certainly be dealt with once the new stellar evolution model is released. Currently, the hard limit to how massive a star can be before automatically going supernova is 11.5 solar masses and it should be the same for everyone. It's a flaw with the current model. The only way to make a more massive star is to turn off the "Radius From Composition" button and then up its mass, at which point the star is no longer a realistic representation.
If you spawn a star like R136a1, turn off "Radius From Composition" and then change its mass to 1000 solar masses, it will not explode. However, at the same time, stars as big as this are not realistically modelled in the first place.