Hi Vanderhoven,
We had a thread over at DtT (here) that includes links in the OP to a couple of related threads. The thread is on the subject of "immortality."
I've come to believe that WT's take on the term amounts to an "etymological fallacy." If one compares its use in 1Co 15 you will see that it is set in opposition to corruption and mortality. Those terms are only used in connection with sinful humans, not sinless creatures.
A related passage is 1Tim 6:14-16 which speaks of either God or Christ as "the one alone having immortality." If it applies to God, then, it would imply that He alone was indestructible in the ultimate sense.
When the WT treats the word according to its literal etymology, they end up creating all kinds of questions that have no easy answer.