Maybe they can go visit Gomez’s Hamburger.
How do you figure that it’s “the same amount of light”? If we were inside the nebula, we’d be much closer to it than now (obviously), so we’d be getting a lot more flux from it. It seems to me that, at a first approximation, it’d be the surface brightness density which would be constant, not the net brightness, in which case the whole sky would look like that little patch of sky does now.
Don’t forget, there’s a star inside that nebula (you can see it as a distant bright light in a few shots during the “submarine” battle). That may be lighting it up somewhat.
In any event, the climax of Wrath of Khan (already by far the best movie) remains the best Star Trek battle. It has all the dramatic tension it needs and doesn’t hinge on some techno-babble nonsense. Rather, it comes down to who is the better ship commander. Contrast this to Wesely Crusher, who (like Khan) had a higher intelligence, but was never faced with a problem in which the more experienced people around him had the solution. Instead, he’d pull this week’s science project out of his ass and save the day.
For example, on this page, shots 25, 26 and 27 show the light generated by the star.