That first quote from DeWitt ("He so interpreted the significance of infinity as to extend it from matter and space to the sphere of VALUES - perfection and imperfection...") I think is purely DeWitt's suggestion, if the text is only this:
“Surely the mighty power of the Infinite Being is most worthy our great and earnest contemplation; the Nature of which we must necessarily understand to be such that everything in it is made to correspond completely to some other answering part. This is called by Epicurus ισονμία (isonomia); that is to say, an equal distribution or even disposition of things. From hence he draws this inference, that, as there is such a vast multitude of mortals, there cannot be a less number of immortals. Further, if those which perish are innumerable, those which are preserved ought also to be countless.
In talking about "values" I think DeWitt is making a reasonable guess, but as I see it the paragraph breaks down into four observations:
1 - the Nature of which we must necessarily understand to be such that everything in it is made to correspond completely to some other answering part. - everything has a corresponding answering part (?)
2 - "This is called by Epicurus ισονμία (isonomia); that is to say, an equal distribution or even disposition of things." - equal distribution
3 - From hence he draws this inference, that, as there is such a vast multitude of mortals, there cannot be a less number of immortals.
4 - Further, if those which perish are innumerable, those which are preserved ought also to be countless.
My reading of these points is that we see things here on earth exist on a scale of COMPLEXITY and/or "SUCCESS" in their achievements. For examples worms on one end and men on the other, on the scale of living beings, minnows vs dolphins, etc. This is hard evidence of a scale of progression in things like acuity of sight, acuity of hearing, physical abilities, and mental abilities.
I gather that Epicurus argued that from this scale of progression here on other it is proper to infer that that scale extends higher in other parts of the universe where life exists. Given that the universe is eternal in time and infinite in space, we should expert the scale of progression to extend these complexities and accomplishments to what we would consider an extreme degree. At the higher end of the scale of progression we should expect to find beings that are far higher in complexity and ental and physical success in humans. And as our human goal is to live as long as possible, and to live in as much pleasure/little pain as possible, it is to be expected that somewhere there are beings which have succeeded in those fields to the point where they are both deathless and painless. And that even though we might not be able to see these beings with our own eyes in the light of day, we should deduce that they exist from the things that we do see in the universe, just as we deduce (on the simple/primitive end of the scale) that atoms exist without seeing them. So in that way inferring the existence of deathless and painless beings is just the flip side of the process of inferring the existence of atoms.