"Did" often replaces any verb, but it also emphasizes, intensifies, makes more real, any verb.
Did you go there? I did.
Yes, I went there
I did go there.
Truly, indeed I went there
They thought spinach had ten times more iron than it did.
They thought that spinach had ten times more iron than it actually does, in reality: if the true value is 35 ppm, they thought the figure was 350 ppm.
(2) "Since when" is very loose. "Since which time" is better, but best of all is to revise the punctuation and wording:
The book was written in 1946; since then, the education system has witnessed great changes.