So who can claim that the YHVH never appeared
in the original writing of the New Testament at least in every place where Old
Testament Scripture are quoted? And who can claim that the YHWH was not in
additional places in the New Testament and not just where Old Testament
Scripture are quoted?
No one can. At the same time, no one can claim it was there, either. That's kind of the point, isn't it? Neither side can prove, beyond doubt, their claim. Of course, now we're getting into the topic of whether the NT should have YHWH in it or not. Based on other people's research, none of the ~5000 extant manuscripts of the NT had YHWH in it. You'd think at least one would, but none do, according to multiple people who have researched this topic. (The shortened form of God's name did appear, as you mentioned. I believe it was twice in Revelation, but I could be wrong about that.) The Greek Septuagint is a Greek translation of the OT, as it was written in the 2nd or 3rd Century BCE. There is some evidence that the Septuagint did indeed contain the tetragrammaton. See here and here.
Personally, I'm not entirely sure what to think of the entire debate of whether the tetragrammaton should or should not be in the NT. It SEEMS reasonable that where the OT is quoted, it should be there. But, what of the places where it is NOT quoted? One of the major criticisms of the NWT is this exact point. Of the 237 places in the NWT where Jehovah is inserted, according to this write-up, only 112 are actual quotations from the OT. That entire site appears to be devoted to exploring the use of the tetragrammaton in the scriptures, with special focus on the NWT.
All this said, I'm not sure what point you're trying to prove.
Is the English translation of God's name Jehovah? Yes. Is that in dispute. I don't think so. Is it the closest pronunciation? No. Are there closer, more probable pronunciations? Yes. Does anyone know the real pronunciation? No. Is this worth getting worked up over and dividing into two camps? Not in my opinion.
The real issue, as I and others have stated, is that since there really are closer, more probably pronunciations, and since JWs have taken it upon themselves to restore Gods name in their bible, and since they are quite vehement about the use of God's name, then why choose the one that is simply a translation instead of a closer, more probable explanation, simply because it's "widely accepted?" Considering that most of their doctrines and practices are not widely accepted, this particular choice makes little sense. I have personally even heard JWs (my own mother, even) deride others who prefer Yahweh over Jehovah. Given that Yahweh is one of the closer pronunciations, that makes very little sense to me, either.
My take on it is this: call God by the name you're comfortable with if it's acceptable. Just don't push the idea that your way is the only way. (By "your", I mean, in general, not specifically you, johnamos.)