I do not think any of these issues are going to change your father, so you may need to find another way to open his mind.
In regards to the word Jehovah in the NT, to say that it was removed with no evidence existing undermines the entire validity of the bible. If God was unable to ensure his name has been left in the bible or be found even once, what else has been changed, what else is wrong? This is a case of the Watchtower Society changing the bible to support their doctrine, rather than allow the bible to speak for itself.
There is ample proof that it never appeared in the New Testament
The Tetragrammaton does not appear in the New Testament either as YHWH or the Greek transliteration Iabe. It has never appeared in the New Testament in any discovered original Greek manuscripts despite the New Testament being one of the most attested ancient works in existence. The Journal of Biblical Literature Vol. 87 p.184 has listed a total of 5,255 known New Testament Greek fragments. Even in the Watchtower Society's own Kingdom Interlinear the name Jehovah does not appear in the New Testament. It is important to realise that it is not a matter of being in some but not others, it is not in a single one of these manuscripts, despite "some papyrus fragments of the Christian Greek Scriptures that go back to the middle of the second century." (w82 3/15 p.23). The Watchtower publication Equipped for Every Good Work (1946) p.58 shows how complete the New Testament record is by saying;
"However the papyrus manuscripts brought to the light of day during the nineteenth and twentieth centuries fill in what was once a blind spot in the chain of preserved Scripture copies. They bridge over the gap of the second and third centuries."
The word Jehovah was not being actively used by the first century CE. It appears that the name of YHWH had stopped being used by the Babylonian invasion over 500 years prior to Jesus.
As early as 250 B.C.E The Septuagint Greek translation of the Tenach was affected by the law on Gods name. For example the Hebrew text of Leviticus 24:16 was changed from: "And whoever blasphemes the name of YHWH shall surely be put to death…" to "And he that names the name of the Lord, Let him die the death…" The Septuagint replaced the Tetragrammaton with "Kyrios". The New International Version translators state in The Making of a Contemporary Translation CHAPTER 9: YHWH Sabaoth: "The Lord Almighty" Kenneth L. Barker
"The Greek word kyrios. The latter is properly a Greek adjective meaning "having power or authority"; used as a noun, it means "lord, sovereign, master, owner." This is the standard word for "Lord" in the Septuagint (the ancient Greek translation of the Old Testament) and in the New Testament.
Jesus regularly quoted or paraphrased passages from the Old Testament. From the words Jesus used when quoting, or more regularly summarising scriptures from the Old Testament, it appears Jesus used several different sources including the proto-Masoretic text and the Aramaic Targum. He predominantly used the Greek Septuagint (LXX) which replaced YHWH with the word for Lord, Kyrios. There are some Septuagint versions with YHWH in it but these are rare.
When quoting Hebrew sources Jesus is unlikely to have uttered the word YHWH out loud as it was illegal to do so. Consider what would have happened if Jesus did utter this sacred term. Jesus would have been in direct conflict with sacred tradition and the law and been accused of blasphemy by the Pharisees. Just as the Pharisees attempted to have Jesus arrested for blasphemy for calling himself the Son of God, they would have had him likewise arrested for illegally using the divine name. Yet Jesus was never accused of using the name. If Jesus had uttered God’s name the furore caused would have been highlighted in the Scriptures.
That the holy Name was not being uttered in Jesus day is attested to by first century historian Josephus:
"…Whereupon God declared to him [Moses] his holy Name,
which had never been discovered to men before;
concerning which it is not lawful for me to say anymore…. "
(Josephus; Antiquities 2:12:4)
Further indication that the divine name was not used by Jesus and did not appear in the original manuscripts is that the earliest of the Ante Nicene Fathers did not use the name. Justin Martyr converted to Christianity around 150 A.D., a mere 50 years after the Bible was completed. He likely had access to some of the first copies of the New Testament. It is interesting to read what he says. In The Second Apology Chapter VI he wrote;
"But to the Father of all, who is unbegotten, there is no name given. For by whatever name He be called, He has as His elder the person who gives Him the name. But these words, Father, and God, and Creator, and Lord, and Master, are not names, but appellations derived from His good deeds and functions."
Justin Martyr shows that no name was in use for the Father, just terms. He was not using the word Jehovah because Jesus and the original Christians had not used it.
QUOTING HEBREW
It is purely speculation to say that when quoting from Hebrew they said Jehovah. There is not a shred of evidence to support this whatsoever.
J VERSIONS
Your father mentioned other translations that have included the word Jehovah possibly from statements made by the Watchtower that draw on the J versions that have included the word Jehovah in the New Testament. The interesting thing about this is that these are translations of the Greek Scriptures into Hebrew, but they were not done until 1385 onwards, over a thousand years after Jesus death, so not proof at all. These texts appear to have been done by Trinitarians, as they use the word Jehovah in many places clearly referring to Jesus. For this reason the NWT does not include the word Jehovah in all the places that the J versions do.
Why Not used by Jesus
The Bible does not specifically explain why the New Testament does not include the Tetragrammaton; however there are several possible reasons.
YHWH was very much a Jewish name. Exodus tells us that the Name was only revealed at the time of Moses, two and a half thousand years after the creation of Adam.
Ex 6:3
"And I used to appear to Abraham, Isaac and Jacob as God Almighty, but as respects my name Jehovah I did not make myself known to them."
God did not feel it important for of the faithful men of old to use his name, the term God Almighty was how he wished to be referred. It was only to the Jewish nation that he provided this identifier.
By the time of Jesus the word Jehovah was also no longer in common use by Jews who feared taking it in vain. By also not using it Jesus showed that he did not want to be offensive to Jewish seekers of truth.
Being a Jewish word using YHWH may have also offended the Gentiles, as it would have made Christianity appear to be a Jewish religion. This would have inhibited the growth of Christianity throughout the inhabited world.
However reading the words of Jesus show the most important reason uttering YHWH was no longer necessary. How did Jesus instruct the disciples to refer to God? Jesus instructed his followers to address God as Lord or Father (Mt 6:8-18, 7:21, Mk 14:36). Jesus referred to God repeatedly as either our Lord or our Heavenly Father and so Lord or Father is the way Jesus followers should refer to God if they wish to follow Jesus example. In the Lord’s Prayer Jesus said to pray "Our Father in the heavens". Can you remember a prayer at the Kingdom Hall that did just that, and did not add the word Jehovah?
To be able to refer to God as our Father is a wonderful privilege. Does a child normally refer to their father by their first name? No, the father-child relationship is far more loving and intimate than that. Once Jesus had revealed the truth about Jehovah it meant that Christians could enter a relationship with God based on a real depth of understanding and intimacy, and hence refer to him as their Father. Repeatedly Jesus used this term and instructed his disciples to do so as well.
Effect of adding to the Bible
Inaccurately inserting the word Jehovah into the New Testament changes the meaning of Jesus message to his followers in a number of ways.
Even in the New World Translation there is no scripture with the statement that Jesus glorified God’s name ‘Jehovah’? Jesus continually referred to God as Father, even when talking about his name. Not once did he use the term "Your Name Jehovah". Notice Jesus following statements:
(John 12:27-28) 27 Now my soul is troubled, and what shall I say? Father, save me out of this hour. Nevertheless, this is why I have come to this hour. 28 Father, glorify your name." Therefore a voice came out of heaven: "I both glorified [it] and will glorify [it] again."
Jesus was glorifying the reputation of the Father. When someone says 'a name is better than gold' they refer to reputation not the word.
Inserting Jehovah has taken away from what Jesus message was, that was his ransom.
Mark 9:38-40
"Teacher," said John, "we saw a man driving out demons in your name and we told him to stop, because he was not one of us." "Do not stop him," Jesus said. "No one who does a miracle in my name can in the next moment say anything bad about me, for whoever is not against us is for us. NIV Zion’s Watch Tower October 1888
"It was an evidence that those Corinthians, who took the party names, had never really appreciated the oneness of the Body of Christ; that they did not really appreciate that Christ is the only head, leader and standard; and that his is the only name by which his followers should recognize themselves and each other…. All true teachers are not only sent by Christ but receive their instructions from him; and any man who attempts to put his own or any other name upon all or any portion of the church is an opponent, an adversary to the true and only Lord and Head of the church."
Jehovah’s Witnesses claim that when they are persecuted in the Name of Jehovah that they are the fulfillment of prophecy. Once again there is no New Testament Scripture that makes this statement. Jesus said"
Matthew 24:9 "Then people will deliver YOU up to tribulation and will kill YOU, and YOU will be objects of hatred by all the nations on account of my name.
For instance see how the following article deflects from Jesus to Jehovah;
Watchtower 1959 October 1 pp.582-583
"Yes, "there is no salvation in anyone else, for there is not another name under heaven that has been given among men by which we must get saved"; and that applies both to salvation from impending destruction and salvation to eternal life.—Acts 4:12. … Today many are delaying to dedicate themselves to Jehovah God and to symbolize that dedication by water immersion."
The Bible makes it clear at Acts 4:12 that it is the name of Jesus that results in salvation, yet the Watchtower quotes this scripture and then goes on to mention salvation coming from dedication to Jehovah. True Christians need to feel comfortable giving Jesus the rightful honour that he deserves as a God and our saviour.
As the name implies, Jehovah’s Witnesses predominant focus is on preaching about Jehovah. However, as Acts 1:8 shows, the message after of the New Testament was to be witnesses of Jesus;
"but YOU will receive power when the holy spirit arrives upon YOU, and YOU will be witnesses of me both in Jerusalem and in all Ju·de´a and Sa·mar´i·a and to the most distant part of the earth."