As to the natural laws possibly they do arise from the qualities too, but to the extent that the qualities are the things observed by us, which are transient, I think we are primarily talking about arising from the properties of the atoms.
As to the number of mortals vs immortals, we don't have much in the texts to go on, do we? I was presuming that Dewitt is inferring from the texts that they would be approximately equal, but on the other hand he does stress "equitable" doesn't he, and it is hard to know what "equitable" would mean here. But I guess it is somewhat clear from what we observe here on earth that we don't have the same numbers of all types of animals. So there may be some kind of parallel there -- we have a few more ants than people, if that tells us anything.