...And we previously saw that the nebula was subject to "lightning storms", so this source of light was probably one of those.
But several facts remain:
a) Genesis was designed to convert a dead planet into a live one, not to build planets
b) Genesis was not flexible, according to its makers
c) Khan showed no skill or interest in reprogramming Genesis anyway
d) The Mutara nebula was right next to the star system where Regula orbited an unknown star (possibly named Regula, too) - that is, within a few impulse-minutes of it
So, when after the detonation we see a living planet next to a star, the obvious assumption is that it's the same old star we are already familiar with - and even the same old planet. If a new star were created, we'd have to explain what happened to the old one!
There are some interesting traits to the eventual Genesis planet that would (quite unintentionally, of course) be consistent with it being the tiny Regula. Sunrises and sunsets are very fast, and the day cycle is short; also, the horizon seems awfully close in the "icy wastes" and "lava hellhole" matte vistas.
Interestingly, the final sunset before the showdown between Kirk and the Klingons falls within the exact same stretch of the horizon where the final sunrise
also takes place during Kirk's foot-in-the-mouth(-of-Kruge) moment! The planet seems to wobble an awful lot...
It would be unethical, since it harms life, no matter how small and it would taint the results.
I didn't notice traits in the Marcuses that would suggest they'd find it unethical to kill microbes. David seemed eager to kill higher lifeforms with his (almost) bare hands, and Carol was okay with dating a professional murderer and having his child.