In my Talmud quotes compilation, I do name the primary two other candidates who contributed to the stock figure "Yeshu" besides Yehoshua of Nazareth. They are Yeshu the Student who served the rabbi ben Perachiah (a rabbi who is yet another Yehoshua) in the early 1st century BC, and Yeshu son of Pappos who was present during the bar Kochba rebellion in the 130s AD. Simple historical inquiry indicates that these were three different people nicknamed Yeshu who fell into disfavor with much of Jewry and so any reference to Yeshu alone is generally an amalgamation of those. In the whole Talmud, besides generic narratives, there is basically one narrative each where a figure can be historically placed, and the narrative for Yehoshua of Nazareth is pretty accurate (he was killed on Passover for stirring up the people, his disciples like Mattai continued teaching his words and were killed for it).

After years of seeing all kinds of things said here about the Talmud, I looked for myself and found the historically accurate parts are pretty easily separated from the exaggerations.

Your reference to ignorance of whatever would be "Lev. 10:31" (you might mean Deut. 18:10-11) seems to mean that Onkelos was not Levitically pure. That's part of the humor of the intended joke, it starts with a taboo before it gets worse and develops its punchline (about how Israel is beloved even in hell). This is spoken of Onkelos when he was a Gentile thinking of conversion, and he never became a rabbi (though he was a translator whose textual work is still used by Christians and Jews). So the point is that even with Onkelos being in total ignorance, he yet received a (humorous) confirmation of the rightness of following Moses, from Yeshu himself, whichever dishonorable Yeshu or Yeshus the author intended.

You may be interested in extended discussion about how Onkelos, being a Gentile convert, is actually my best historical candidate for who Jesus meant by the synagogue of satan. He had fingers in many pies.