aqwsed : I think it follows from [textual variation] that there must have been "Jehovah" in all such places. Well, that's a pretty unscientific proposition.
Of course I never made that proposal. I was specifically addressing Jude vs 5 and was proposing 'Jehovah' as a possible cause of the variation in that verse. When addressing variation in a text there are certain common principles that can be observed but each text is different and it does not simply follow that the cause of variation will be the same. One common principle considered is whether the text contains a quotation of the Old Testament which includes God's name, and in these cases greater weight can be given to the probability that the use of God's name in some form by the writer gave rise to variation among the copyists. In the case of Jude vs 5 it alludes to Exodus 12:41 and Numbers 14:35, both of which contain God's name. When the reading ("Jesus") is "difficult to the point of impossibility" (UBS Committee) you have to consider what may have caused the variation. It is unlikely that a copyist would replace "God" or "Lord" with "Jesus" in a verse referring to God's deliverance of Israel from Egypt, which leaves you with the plausible option that the text contained God's name in a form that Greek copyists did not understand.
Regarding the passages in the Tosefta that specify that the gilyohnim and the books of the minim which contain the names of God do not have to be saved from fire, I accept that gilyohnim may not refer to the gospels specifically. You say that the books of the minim refers to the writings of the Jewish heretics (e.g. Sadducees, Nazoraeans, Samaritans etc.), but it is not clear why you exclude those Jewish heretics and their writings whose "heresy" was belief in Christ. At any rate, I am pleased to note that you now accept that the minim (whoever they were) did use the names of God in their books, which certainly included the tetragrammaton as the context shows.