Yeah, the whole "new light" idea is bogus. The Society invokes it when they change their doctrines, sometimes back and forth, pretending it's God's doing.
It's interesting what the Watchtower had to say on this is Russell's time:
"If we were following a man undoubtedly it would be different with us; undoubtedly one human idea would contradict another and that which was light one or two or six years ago would be regarded as darkness now: But with God there is no variableness, neither shadow of turning, and so it is with truth; any knowledge or light coming from god must be like its author. a new view of truth never can contradict a former truth. new light never extinguishes older light, but adds to it. If you were lighting up a building containing seven gas jets you would not extinguish one every time you lighted another, but would add one light to another and they would be in harmony and thus give increase of light. So is it with the light of truth; the true increase is by adding to, not by substituting one for another." (Watchtower Feb. 1, 1881, Reprints p. 188)
Well, the Society then went on to have a long history of constantly "contradicting former truth" and "extinguishing older light", which according to this Watchtower is evidence that JW's must be following men and that their changeable literature is full of "human ideas", so they are condemned out of their own mouth (as is usually the case).
As you said, if they aren't 100% sure of their interpretations - which, after 100 years of getting things wrong, you'd think they'd learn some caution and humility - then they have no business claiming or implying that their present interpretation has been revealed by God nor of requiring that all JW's accept them as the truth without question.
Further, if you read the context of the Scripture from which they lift the notion of 'light' getting 'brighter and brighter' it doesn't even have anything to do with doctrine or gradual understanding. The Scripture is completely misused to try to excuse their constant errors and ever changing doctrine and to pretend it's all part of God's plan.
It's hogwash.