Apostate - No

James H. Moyle visited David Whitmer on June 28th, 1885, this is part of the conversation:

While I am speaking from memory, the foregoing most essential facts to me and which are the sole object of my visit, were so indelibly impressed upon my mind, that they are perfectly clear and unmistakable. I asked him many question concerning his experiences with the origin of the Book of Mormon, and why he left the Church, all of which he endeavored to answer frankly. We talked for a considerable time. His mind seemed clear and his mentality above rather than below the average. He said he had never left the Church, that they had maintained a branch of the Church in Richmond, [Missouri], and that he had always been active in it. (James H. Moyle in Conference Report, April 1930, p. 122)

Martin Harris told William Harrison Homer something similar:

It was a sublime moment. It was a wonderful testimony. We were thrilled to the very roots of our hair. The shabby, emaciated little man before us was transformed as he stood before us with hand outstretched toward the sun of heaven. A halo seemed to encircle him. A divine fire glowed in his eyes. His voice throbbed with sincerity and the conviction of his message. It was the real Martin Harris, whose burning testimony no power on earth could quench. It was the most thrilling moment of my life.

I asked Martin Harris how he could bear such a wonderful testimony after having left the Church. He said, 'Young man, I never did leave the Church; the Church left me.'

Martin Harris never denied the faith, never affiliated with any other sect or denomination, but when the Church came west, Martin Harris remained behind. It is true that Martin Harris did not apostatize; he was never tried for his fellowship; he was never excommunicated.(William Harrison Homer, "The Passing of Martin Harris," The Improvement Era, vol. 29, March 1926, pp. 468-472)

This from the Rochester American:

A MORMON APOSTLE.—We received yesterday a visit from Martin Harris, formerly of Palmyra, who was concerned with Joe Smith in originally proclaiming the Mormon faith. He wrote the book of Mormon from Joe Smith's dictation, the latter reading the text from the golden plates by putting his face in a hat. When the volume was written, Harris raised funds for its publication by mortgaging his farm. But he no longer goes with the Mormons, saying that they "have got the devil just like other people." He abandoned them fifteen years ago, when they assumed the appellation of "Latter Day Saints," and bore his testimony against them by declaring that "Latter Day Devils" would be a more appropriate designation.

Mr. Harris visited England some three years ago. At present he professes to have a mission from God, in fulfilment of which he wanders about preaching to "all who will feed him." When this essential condition is not performed by his hearers, he shakes off the dust from his feet, and leaves for more hospitable quarters. Mr. H. is exceedingly familiar with the Scriptures, and discusses theology, in his peculiar way, with the fluency and zeal of a devotee.—Rochester American. (DAILY EVENING TRAVELLER, Boston, Wednesday evening, November 21, 1849 [V:198]; Portand Transcript, December 1, 1849; National Intelligencer, November 22, 1849)

Martin Harris never denounced the Book of Mormon, but like Whitmer, he held others to the flame of its truth. Had Harris not gone to Utah, the followers of Brigham Young may have lessened their interest in the Book of Mormon like the Reorganized church did.

BAPTIZED OR REBAPTIZED FOR SINS?

It's assumed that Martin Harris and Oliver Cowdery were "rebapitzed," having fallen from the Mormon Church into "sin." The truth is, the average Mormon was baptized multiple times:

There seems to be a kind of standing ordinance for all Latter-day Saints who emigrate here, from the First Presidency down, ALL ARE RE-BAPTIZED and set out anew by renewing their covenants. (Orson Pratt, Journal of Discourses, 18:160)

Thus, the re-baptisms of Oliver Cowdery and Martin Harris were the same as other Mormons.