I think you are being unnecessarily pedantic here and are placing conclusions on Jonsson's words that were certainly not originally intended.
1. He does not say that the 70 year period of Jer 25:11 ran from 609-539 BC. He concedes that it may have done so, but it could just as easily have been a typical Middle Eastern figure of speech current at the time for an approximation of 70 years. If the period began in 605 BC and ended in 539 BC then we have a period of some 66 years which as I said is an approzimation. The point is that no one knows the exact date for the starting point. Not the Watchtower, not you.
2. The contention behind Jer 25:11 is misplaced by you. The emphasis is on what the two statements made in that text mean, and what the Watchtower makes it mean. According to the NAB translation, Jer 25:11 says:
"This whole land will be a ruin and a desert. Seventy years these nations will be enslaved to the king of Babylon"
Jonsson's contention is that the Watchtower reads this text as: "This whole land will be a ruin and a desert [for] seventy years. These nations will be enslaved to the king of Babylon". The "seventy years" mentioned here answers the question: "How long will the nations be enslaved to Babylon?" and not, as the Watchtower interprets it, "How long will this whole land be desolate?" The first sentence has no time limit, the second does.
As one can see, despite the fact that Jer 25:1 makes it clear that Judah and Jerusalem are being addressed, it is equally clear that, as far as verse 11 is concerned, the nations around about are included in a specific time period [70 years] and prophecy [enslavement].
3. The Neo-Babylonian conquest under Nabopolassar, actually began in 612 BC, when Nineveh, the capital of Assyria fell to the invading Babylonians. The inhabitants were enslaved, but the Assyrian Empire barely survived by moving its capital to Harran. By this defeat, much of Assyria's fighting force were enslaved in Babylon. But in 609 BC the Assyrians suffered another major defeat and Harran was captured by the Babylonians.
Assyria was now so weak that it no longer could fight off the Babylonians without allies. The capital was again moved, this time to Chargemish, and here in 605 BC the final Assyro-Babylonian battle was fought. The inexorable tide of Babylonian conquest was riding athwart the entire ME area. Nothing would stop them. The Assyrians and their Egyptian allies lost, and Assyria disappeared from off the map. Suffering the same fate it had meted out to others.
4. The use of the prophetic element in Jer 25:11 is both post positive as well as future. And not just one way as you suggest with your Hebraic techno-babble. By 605 BC elements of Jeremiah's prophecy had already began its fulfillment, but the major portion, still future, would stand as a sign of Judah's continuing apostasy till 539 BC.