berrygerry,
When you look at the first diagram at that site you referenced in your initial post, you will note very carefully that in accordance with the WTS's "explanation", that the apologist correctly identifies their start of the "70 years" as "when Judah went into exile from off its soil", dating this in the 7th month (October, Tishri).
The WTS has two reasons for focusing on the exile into Egypt in the 7th month: firstly, it ties in with their termination of the period in the 7th month. Equally significant is the fact that Jerusalem was devastated in the 5th month.
I have analysed the situation, and it is impossible for the "exile into Egypt" as taking place only two months after the destruction of Jerusalem. I suspect that at least 2 years passed and possibly 4 years.
http://www.jwstudies.com/Did_Jews_exit_after_two_months.pdf
In addition, the WTS is incapable of arriving at 537 BCE for the Returnees to assemble at the site of the destroyed temple, or that any Scripture says this marked the end of the period. 2 Chr 36:21-22 makes it clear that the period of Babylonian rule ended on the day it was defeated (539 BCE).
And the WTS gets its 539 BCE date from the secular sources who use processes that the WTS rejects.
And if it needed the land to be emptied before the "70 years" could commence, why does the WTS not apply that "principle" to its end? The Jews returned to the temple site long after each had been settled into theor own villages.
Doug