What would happen if it was proven "Jehovah" did appear in the NT? Very little. Most Jehovah's Witnesses are so poorly informed, they already believe it is proven that it did appear in the NT, they are completely unaware of the dishonest way the subject has been presented by Watchtower.
People don't belong to religion because they care about whether the God's name is Jehovah, Allah or Geoffrey Jackson. They worship for the reward of an afterlife. Jehovah's Witnesses want the New System, and they would hang around for that carrot, whether YHWH is in the New Testament or not. They will hang around for paradise if the name is changed to Yahweh; even if Jehovah becomes Triune.
The bigger issues for Watchtower is the never ending delays in their doomsday predictions. The members are aging because young ones see little motivation in prophecies that refer to the 1900's, and are far more aware that the Bible itself is not inspired.