A refreshing and no-nonsense review of the authenticity question was given by William Webster in his The Greek Testament with notes...(Princeton, 1855). A conservative scholar, with little concern for the confusion of previous commentators, provides a good overview on page 442 fwd:
"The external evidence for and against the genuineness of this passage, derived from the number and value of the MSS and versions in which it is contained or omitted, is very nearly equal. For a full account of it, see Bloomfield's Recensio Synoptica. The internal evidence preponderates greatly in its favour as genuine.
(1) No real discrepancy can be pointed out between its style and the usual style of the Evangelist. On the contrary, see 2, 4, 6, 9, n.(2) The conduct of our Lord on the occasion here recorded is in accordance with his general conduct, with his character, and with several of his express declarations; so much so, that it might be adduced as a good instance of undesigned coincidence with them (See v. 15. 3:17, 12:47, L. 9:56, 12:13-15, 19:10. )(3) At the same time, his treatment of the accused woman was obviously liable to be misunderstood, and would present many difficulties to the earliest commentators on the NT. Their views of the nature and object of Christ's mission, and of the distinction between the covenants of the law and the Gospel, were imperfect and limited. This was the result of the prejudices of their philosophical and religious education, whether Jewish or Gentile. And they were all more or less embued with the spirit of that Gnostic asceticism which afterwards so extensively prevailed. Hence this passage would be a stumbling block to them which they would be glad, on any pretence, to remove.To say the least, its omission by them is much more easy to bet accounted for than its introduction at a later date. For if the story appeared improbable, from moral considerations, to expositors of the 3rd and 4th century, it would appear far more so, on the same grounds, to those of the 7th and 8th. The former were more likely to pass by unnoticed, and their contemporary transcribers to omit, a narrative not found in all copies of S. John's gospel, and difficult to reconcile with their notions of the character of Christ and Christianity, than the latter were to insert in the sacred books a floating tradition, which had not previously appeared in them, and which their known opinions would lead them rather to reject than to receive.For the principles on which the rejection or omission of this narrative may be accounted for, see Dr. Wordsworth's Lectures on the Apocalypse, Lect. I, pp. 12-15.(4) The contrivance of the Scribes to find matter of accusation against Jesus is in accordance with the attempts related in L. 20, viz. the enquiry concerning his authority, and the question about the tribute money. And the subject, on this occasion, is nearly allied to that of divorce which the same parties brought before him, 'tempting him'. M. 19:3, Mk. 10:2.(5) His answer is of the same character as in the two former cases, creating a difficulty or dilemma, (see also L. 20:44.). And yet the similarity is not so close as to suggest the idea of imitation by a pseudo-Evangelist. Moreover, the conduct of our Lord in teaching by action on this occasion, corresponds with the instances of his setting a little child in the midst of his disciples, and his washing their feet.(6) Each of the other three Evangelists has related, or alluded to , the attempts of the Scribes and Pharisees above mentioned. Such attempts, therefore, were particularly important occurrences in the life of Christ. Hence it appears natural that S. John should relate one; and it is in entire consistency with the general character of his Gospel, that he should pass by those described by the others, and record one not referred to by them. See also on v. 15 and v. 20."