The question is, "How is the New World Translation's usage of 'Jehovah' controversial?" The concluding question is similar: "In what way is the translators’ decision controversial?" This logically refers to their decision to render the Tetragram as 'Jehovah' in their English translations.
However, the other three questions fitted in between those bookend questions risk skewing answers to deal with what is supposed to be directly connected to the main question, when they are not.
There is nothing controversial about why the NWT chose to transliterate the Tetragram the way it did. Others had previously done that, and their translations are not generally viewed as controversial for that reason.
There is nothing controversial about why the choice was made to use the transliteration 'Jehovah' rather than Yahweh or another word. Again, different translations have used Yahweh and nobody objects to that.
There is nothing controversial about why the name 'Jehovah' might be viewed as conveying the correct information (in and of itself, as long as it's not used as some sort of marketing device.)
Therefore, the only question that must be tackled head on, without any distractions, is why others may view the NWT as controversial, the way it uses the name 'Jehovah'. The Watchtower Society was not the first to come up with the word 'Jehovah' as an English representation of YHWH (in Hebrew). They did not invent that. So, no controversy can be found there. Nor is it controversial that they should choose to render it thus all of the nearly 7,000 times it appeared in the ancient Hebrew Scriptures. That is simply being consistent. A controversial point does arise, however, with inserting it 237 times in the Christian Greek Scriptures. If the claim that the NWT had "restored" it to the original text was true, then it would have had to have been in there in the first place, being subsequently removed. Let me now quote from the author of The Tetragrammaton and the Christian Greek Scriptures who delved into this, and related matters, regarding the Divine Name:
"A translator cannot 'restore' a new word to his translation if that
word is not found in the Greek text he is translating. The Watch Tower
Society admits that there are no ancient Greek manuscripts of the
Christian Scriptures which contain God's name in Hebrew letters [but
claim their removal as a result of a great heresy in the second and
third centuries C.E. - see link following.] We also know that there
are no ancient Christian Scripture manuscripts which contain a
transcription of the four Hebrew letters into Greek letters.
A Textual Commentary on the Greek New Testament, United Bible Societies, 1971, lists no variant (an alternative reading which
differs from the wording of a majority of Greek manuscripts) of God's
name for any of the 237 NWT Jehovah verses. However, there are
ancient Hebrew Scripture manuscripts (not Christian Scripture
manuscripts) and other religious writings from the same time period
that do contain God's name transcribed into Greek letters or into
transcription-equivalent Greek letters (PIPI)." The Divine Name in
the New World Translation, pp 3-4, Lyn Lundquist,
http://www.tetragrammaton.org - free to download with no copyright.
The author then gives due consideration to the two New World Bible Translation Committee proposed translation guidelines, and a third hypothesis regarding the history of the early Christian congregations, used in combination to support their use of Jehovah in their Christian Scripture translations. The subject is massive, so those wishing to thrash the matter out need to read the literature involved. One of his conclusions is:
"Inasmuch as the Tetragrammaton is not used in the Greek Scriptures,
all passages translated as Jehovah in the New World Translation
Christian Scriptures must rightfully be translated as Lord where
Kyrios is found in the Kingdom Interlinear Translation.
The translator must communicate the exact thoughts of the original
writers to his reading audience. The translator cannot become a
commentator, explaining what he (the commentator) believes the
Scripture writer intended to say. A commentator may do that later,
using properly translated Scripture passages. Even the translator may
do that later. But the translator is not free to use his translation
as a statement of personal opinion.
This does not ignore the fact that a translator must often make
subjective choices when translating a word or a phrase from Greek into
English. In many instances, that decision will involve the personal
opinion of the translator regarding word meanings according to the
context. Kyrios was a common secular word in the Greek language
of Jesus' day. It is appropriately translated in the NWT as Sir
(Mark 7:28), owner (Mat.21:40), master (Mat.25:26), a protocol
form of address for an emperor (Acts 25:26), and slave master
(Eph.6:5). However, this does not give the translator the privilege of
substituting one known word for another with an entirely different
meaning as would be the case in substituting the divine name for
...The author holds in high regard those translators who have made the
effort to use a proper translation of [the Tetragram] rather than
LORD. However, inasmuch as the Tetragrammaton is not found in any existing manuscripts of the Greek Scriptures, it is a violation of
inspiration to insert the name where there is no evidence that the
original Christian writers used it." (Ibid. p68)
That is one reason why some view the NWT use of 'Jehovah', to the extent that it does, as controversial. There is another controversial point, which has to do with what could be viewed as a 'vested interest' in taking the words of Almighty God to the ancient nation of Israel in Isaiah 43:12 and applying the phrase 'Jehovah's witnesses' to the early 1900s group that stuck with the Society's second president Rutherford. Their NWT came out more than two decades later, using the English word 'Jehovah' thousands of times, and since then the public has made a direct connection between that religious group, and the word 'Jehovah'. Thankfully, no religious group has a monopoly on the Divine Name, and nobody is trying to 'wrest away' the word 'Jehovah' from the Jehovah's Witnesses. They use it as they wish, and others use it differently, so avoiding the controversy that has become associated with the NWT.
(For clarity: I use the word 'Tetragram' where most other say 'Tetragrammaton'. When I quote, I have to stick to 'Tetragrammaton' in the text I quote from, but I revert back to 'Tetragram' for my own use. The author of the book I quote from did point out that 'Tetragram' is correct but that common usage is 'Tetragrammaton', which he has chosen to go along with.)