But their core teaching remains not only UNCHALLENGED and UNCHANGEABLE
Wrong on both counts, abiathar.
WT teachings are constantly challenged and exposed as untenable ...you may have noticed it happens here everyday.
UNCHANGEABLE
This is especially lauaghable, considering WT itself has changed at least two of its most 'core' doctrines recently: re-defining the identitiy of the faithful and discreet slave (again), and concocting an overlapping generation (this is the 9th redefinition of what "generation" means).