TD - there is no uncertainty regarding the reigns of the king - IF - you bother counting the years properly. And you have to read the scriptures very carefully to do that work.
Once you get to the reigns of Cyrus, Artaxerxes, etc, they are dealt with in an entirely different manner. Their reigns are only mentioned for the purpose of the tiny bit they are used for. Nothing else is allowed to be known because they are irrelvant to the prophecies to do with the coming of Jesus Christ.
The scriptures provide enough into to go from Adam 1 to Adam 2, providing you don't put any dates in and just go by events and ranges of time.
You can debate the 70 years (I'm no expert so I wont go there) but the info is there if you want to work through it.
Once you get to the centre of histroy, ie Jesus Christ, he, Jesus says he'll return at an unknown day and hour.
And yes Watchtower do date the metal image to 606 BCE, and that dream took place in the 2nd year of Neb. Jerusalem fell in the 19th year of Neb which they say is 607 BCE. So Neb's 19th year of rulership is followed by his 2nd year of rulership. Logic of the Watchtower. Only bettered by their saying Jehoiakim ruled for 11 years and died in the 3rd year of his rulership.