I don't see any of the ambiguities you postulate. We have a pretty good idea of how far the nebula is from the Regula rock, say. For one thing, Spock believes they can escape Khan by going there - which already means Spock doesn't think their escape will be due to a long chase in which they outrun Khan. Indeed, this they clearly cannot do, because Khan easily catches up with them and fires a torpedo at them. Almost immediately thereafter, Kirk is so close to the nebula that Khan wisely ponders giving up pursuit, until goaded by Kirk not to.
There simply isn't any plot time for or plot logic in a long, slow chase where Khan gradually catches up. After all, Khan doesn't move at full speed initially. If there were a chase, he'd win it by throttling up. But there is just this surprise pounce from Regula to nebula, during which Khan doesn't have time to think through the implications.
Indeed, the trip from Regula to the final confrontation might well take just the four minutes specified for the escape. But there would be nothing wrong with the distance being more like twenty light-minutes, plenty enough to make the local star basically invisible (although perhaps still capable of creating the odd lighting effects in the nebula, assuming the camera just ramped up the contrast). There is no reason to think the Genesis wave would have its range limited to any specific figure, other than the implicit "It will engulf us if we don't warp but the rest of the universe will still be fine".
You clearly see the nebula vanishing in the movie, so the Genesis device is creating the planet out of the nebula!
Obviously not - since the nebula disappears along with the starship, but nothing is created in its place. We see but ejecta there, on its way somewhere else.
The camera then cuts away from this location, which remains visibly empty. And then we have bridge action. And then
we see the already readymade planet, in an unknown direction at an unknown distance.
It may be at the center of the wave. Or it may be at one of its impact points, just as in the simulation we saw.