This is a more mechanical analysis of your quote. You ask:
Does here "let alone the science" want to say "as we cannot understand these sand paintings and science cannot understand them either"?
No. Here's why.
There are two idiomatic forms used together:
- "cannot X nor Y" = "cannot X and cannot Y"
- "cannot V the N1, let alone the N2," = "cannot V the N1, and certainly cannot V the N2,"
We apply the first, with
- X = read the signs
- Y = subscribe to the belief, let alone the science
so the relevant part can be rewritten as "[we literally] cannot Y ...", or "[we literally] cannot subscribe to the belief, let alone the science ...".
We then apply the second idiomatic form above to this, with
- V = subscribe to
- N1 = belief
- N2 = science
to get "[we literally] cannot subscribe to the belief, and certainly cannot subscribe to the science, ...".
Putting this back into the context produces, as others have observed, the statement that the author cannot subscribe to the science that "she can be made better this way", not that science cannot understand the sand paintings.