Rutherford can't have written Let God Be True since he died in 1942.
But you raise an interesting point about the cut off for the anointed being moved from 1931 to 1935. I've never gotten to the bottom of why that changed or what the reason for either date really was.
Some have pointed out that if they accepted mainstream Bible Chronology and Jerusalem fell 20 years later then they could say the Gentile Times ended in 1935. But since they've abandoned that date now there's no reason to fit it there. And even a "generation" from 1935 is now a bit of a stretch.