To the contrary, nothing in the movie(s) gives support to the idea that Genesis would be capable of creating planets out of nothingness. That's neither what it was created for, nor what it was credited for.
The device has exacting requirements that keep a starship occupied in an ultimately fruitless search for what sounds like months at least. It's not going to mutate itself into a different device altogether (not unless itself struck by another Genesis device at least
Well, given that Regula is clearly not inside the nebula, and given that the detonation of the Genesis Device happens within the nebula, how could the planetoid be caught in its effects?
Why should this be a problem? We never get any indication that the nebula would for be a barrier for the Genesis wave in any manner. Obviously, it is going to reach Kirk's starship unless Kirk goes to warp - so just as obviously, it's going to reach a planetoid that Kirk only very recently departed at damaged impulse.