Moxy is correct about the JW idea of free will, that there are things which Jehovah chooses not to know.
In the JW teaching, those with the heavenly hope are both declared righteous and spirit-begotten. If someone is declared righteous, they have righteousness imputed to them or credited to their account. They may be disfellowshipped, but only Jehovah can remove that righteous standing which he has given them. If they repent before he does so, they are received back as anointed ones. This is the way I understood it as a JW.
Why doesn't he wait until their death to determine who is chosen and who is not? Because there is something about the spirit-begotten condition that enables them to develop toward the goal which He has in mind for them. They cannot develop as "new creatures" unless they already are such in some sense while still in the flesh. The new life must begin here and now, and if they fail, then they become like an aborted fetus. The "sealing" of the 144,000 is not when they are first chosen, but when God decides that they have been fully tested and tried (whether at or before death is not clear). So while the number is supposedly full at the present time, all the individuals that will finally make up that number have not been sealed.
I am only saying that the system seems logical within itself. Of course, with anything, one can always ask why God does something in one way and not in another.
What is putting a strain on the belief system now is the fact that, with the passage of time, it seems that so many have proven unfaithful and have needed to be replaced. Every "replacement" increases the strain. It keeps the anointed class on earth for the final showdown, but at the same time makes it appear that such ones have been called to a hope which is practically unattainable.