Regarding Jeremiah 29:10 and its reference to the exile being foretold to end after 70 years, it should be noted what The Interpreter's One-Volume Commentary on the Bible says. It says the following. "29:10-14. This passage is probably a later insertion, coming from an editor who knew that from the battle of Carchemish in 605 (cf. 46:2) to the Persian capture of Babylon in 539 was roughly seventy years. The intention is to encourage restoration hopes."
The New Oxford New Annotated Bible (a study Bible) says the following regarding Jeremiah 11-12. "Seventy years (cf. 29.10). The period from 605 BCE until the defeat of Babylon by Cyrus the Persian in 539 BCE is sixty-six years. Seventy, however, is likely a symbolic number, representing the length of a lifetime (Ps. 90.10)."
Regarding the WT saying that Jerusalem was destroyed in 607 BCE (instead of in 587/586 BCE), the WT originally said it was in 606 BC. That year is only one year earlier than the 605 BCE date of the first siege upon Jerusalem (at least according to the Bible, see Daniel 1:1-2, and other sources). According to the Bible some captives, including Jehoiakim the king of Judah, were taken at that time into exile. The gentiles can thus be said to have begun trampling Jerusalem at 605 BCE, though Jerusalem was not desolated at that time (though it likely experienced some damage at that time). If the society were to adjust their 607 BCE teaching to 605 BCE and focus upon what happened in that year and if they also said that Jerusalem was destroyed in 587 BCE or 586 BCE, then they would be much more in harmony with secular scholars. Doing such would entail a minor adjustment of their 1914 date to 1916 (or so, depending in which month the siege in 605 BCE took place in), but that would be no more a problem than their existing 1914 date. They might even wish to link it with a comment of Russell having died in 1916. Perhaps they also might go back to saying that Russell spiritually ascended to heaven in 1916 and say he began ruling with Christ in 1916, and say that the timing is highly significant. Doing that would likely require an adjustment to what they say happened in 1918 in regards to their scriptural claims.