Amazing,
I think you are right. Rutherford's writing was much more direct and dogmatic. The writing in 1975 had more loop holes built into it. It is interesting that in 1925, the Society was building new printing facilities, a strange activity for that particualar year.
Refiner's fire,
Like you, I think the driving out of Russell devotees was a major factor in the decline. Also, from Rutherford's presidency until about 1935, he was changing beliefs and proceedures at a dizzying pace. I think it was too much for many to absorb. Also, the memory of the last failed prophecy was only 11 years old by 1925, compared to the 50 year span from 1925 until 1975. I think that two failed prophecies within eleven years was too much for some to take.