I think there can be little doubt about (2 [that Grant has the wrong Celsus in mind]). Nor, I think, is there any doubt that the passage in quotation marks, as quoted, exists nowhere in Greek, let alone in Origen/Celsus. My money, though, is not so much on outright fabrication as free interpretation, perhaps partly based on a passage like the following: Τὸ γὰρ κινῆσαν φθόνον τῷ Ἰησοῦ καὶ διερεθίσαν Ἰουδαίους πρὸς τὴν κατὰ τούτου ἐπιβουλὴν τὸ πλῆθος τῶν ἑπομένων αὐτῷ εἰς τὰς ἐρήμους ἦν, πεντακισχιλίων καὶ τετρακισχιλίων ἀνδρῶν αὐτῷ ἀκολουθούντων χωρὶς τοῦ τῶν γυναικῶν καὶ τῶν παιδίων ἀριθμοῦ. Τοσαύτη γάρ τις ἴϋγξ ἦν ἐν τοῖς Ἰησοῦ λόγοις, ὡς οὐ μόνον ἄνδρας ἕπεσθαι θέλειν αὐτῷ εἰς τὰς ἐρημίας ἀλλὰ καὶ γυναῖκας, οὐχ ὑπομεμνημένας τὴν γυναικείαν ἀσθένειαν καὶ τὸ δοκοῦν ἐν τῷ ἀκολουθεῖν εἰς τὰς ἐρημίας τῷ διδασκάλῳ· (3.10.7-15). It certainly emphasizes what is cast as the particular impropriety of a large following of women in the context of an explanation for Jesus’ persecution.Here is the relevant portion translated from Roberts-Donaldson: "For what stirred up the envy of the Jews against Jesus, and aroused them to conspire against Him, was the great number of those who followed Him into the wilderness,--five thousand men on one occasion, and four thousand on another, having attended Him thither, without including the women and children. For such was the charm of Jesus' words, that not only were men willing to follow Him to the wilderness, but women also, forgetting the weakness of their sex and a regard for outward propriety in thus following their Teacher into desert places."
I think that Mark's suggestion has a great deal of merit. While I do not rule out duplicity, this could easily be explained by free-wheeling proof-texting, followed by transmission snafus. I still think it would be overly generous to call this even a paraphrase. Larry's point about the third party scribe is worth emphasizing too. Perhaps Grant had not intended for this allusion to be rendered as a direct quotation. I would even take it a step further: perhaps Grant had never even read this passage, but remembered that another Mormon preacher appealed to it in support of Jesus' polygamy. This would explain Grant's confusion.
I would love to give Mysterious Mark the obligatory footnote; does he have an email address?