One thing you guys aren't really taking into account is the density of the remnant nebula. I don't have any numbers on how dense supernova remnants tend to be, or how quickly they spread and dissipate, but I do know that nebulae in general tend to be exceedingly sparse, many orders of magnitude less dense than what we experience in FS2's nebula missions. (Obviously,
took some artistic license with the nebula for gameplay purposes.) Even if the nebular gas was at an extremely high temperature, its extremely low density would mean that anything passing through it probably wouldn't be affected at all, especially considering the sort of weapons fire that FS2 ships are built to withstand. A similar principle is at work in a layer of Earth's atmosphere called the thermosphere, as well as within the Sun's corona.