I agree with Anthony Stadlen's larger point, which is that by 1948 it
was impossible to make any assumptions about the identity or
reliability of any Nabokov narrator, no matter how superficially
ordinary or omniscient. In this particular case, this one sentence, I
take the situation to be a variant of free-indirect discourse, where
"fault-finding" reflects the attitude of Mrs. Nameless, or perhaps of
"other women of her age (such as Mrs. Sol. . .)". The narrator seems
to glide seamlessly among available discourse sources (especially those
of "she" and of the doctors; but even these could be filtered through
"she") but without evaluating their veracity. I think that rather than
specify that the narrator attributes moral agency to nature, one might
instead suggest that the narrator draws attention to the fact that
(nearly?) all human beings attribute such agency to nature, in one way
or another, if only in their metaphorical engagement with the world.
Indeed, why should the boy be "totally inaccessible to normal minds" if
his though process bears this commonality with his mother's?
2nd paragraph. Light does not find fault. People, or God, find
fault. To assert that light finds fault is to fall into the Pathetic
Fallacy. Therefore, if the boy is "deranged in his mind" because he
attributes moral agency to inanimate nature, so is the narrator.