The Jews certainly did have a canon. And the Jews today have a canon, with no significant argument today about what their canon is. As far as I know, there was no significant disagreement among the Pharisees and the Greek Jews about that canon, although there certainly was a disagreement with the Saducees!!! (and the Samaritans). The Council of Jamnia is not the invention of the mid-nineteenth century. The Jews themselves described and reported on this council. You say that “no scholar believes that this actually happened. If you like, I can furnish you a list of dozens of scholars who DO believe that it happened. Besides, like I said, as far as we know, this group of Jews only confirmed what was already in place. Also, Josephus lists the accepted books of the Hebrew scripture at almost the same time. His list is identical to that assumed by the Council of Jamnia. He reports that this canon was in place since Ezra. (personally, I believe he is exaggerating here, but…) Also, there is the Septuagint. The Septuagint translation into Greek includes the same list of books that was mentioned by Josephus, and this translation was made by 200 BC. Let us add the Dead Sea Scrolls. Of course, the evidence here is a bit ambiguous, as these scrolls included many of the writings of the Essenes, but if one looks at the DSS, one can see a clear distinction between the Hebrew canon (in Hebrew!) and the writings of the Essenes in Aramaic. Anyway, in the DSS all the canonical Hebrews texts are found except Esther. Add to that fact that none of the Apocrypha are in there. Again, this is solid evidence that the Essenes, as well as Josephus and those who met at Jamnia had an identical canon. Your statement that there were many different canons is simply not true (although there was a dissenting canon of the Saducees). What is your evidence for this claim that there were many different canons? What is your source for this?
Let me add one thing. Perhaps what you read is that recent scholars have tended to doubt, not that the Council of Jamnia occurred, but that it was at this council that the Hebrew canon was set. Although we have accounts of this council, there is some debate about whether the council actually published a canon. I do agree that there is significant doubt about whether they published a canon, but not that this group met in Jamnia. FF Bruce summarized the evidence well: (from Wikipedia) The evangelical scholar, F F Bruce, thought that it was “probably unwise to talk as if there were a Council or Synod of Jamina which laid down the limits of the Old Testament canon.” I agree with this statement. The canon was well established before this group of Jewish scholars came together, and they were only having a limited discussion about a couple of books on the “fringe” of the canon. However, what was not discussed was the rest of the canon, because it had already been authoritatively established for at least two hundred years before this group met. There was little if any room for doubt about this canon. Besides, Jesus, the consummate Jew, quoted from nearly every book in the Hebrew canon, and did not ever quote from non-canonical books. This is further evidence that, although he did not list a canon, what we now call the Old Testament was a stable set of writings by the time of Jesus.