If we ignore the possibility KEP was an after-project. KEP was not very important in the grand scheme of things, nor was it touted in any way, shape, or form. It was not canonized, or used in any exciting way.
As I said, there are several lines of evidence, only a couple of which have to do with the KEP. (Furthermore, your characterization of the KEP project is not accurate.) Briefly,
1) The Book of Abraham itself says Facsimile 1 was "at the commencement of this record". On the original papyrus scroll, it appeared "at the commencement" of column 1 of the Hor Document of Breathing.
2) The Kirtland Abraham manuscripts contain sequential Egyptian characters from column 1 of the Hor Document, matched up with sections of the Book of Abraham text. The Abraham manuscripts reconstruct and translate characters missing from damaged portions of the papyrus, consistent with contemporary reports that Smith could "read the parts that were destroyed equally as well as those that were there." And the large ratio of English text to Egyptian characters agrees with Cowdery's report that the language of the papyrus was very compact. In other words, the relationship between characters and text in the Abraham manuscripts is consistent with what was reported of the translation process at the time.
3) The translations in the Abraham manuscripts agree with the translations in the Egyptian Alphabet manuscripts. Joseph's history and journals give him credit for the Alphabet and Grammar and imply the project was inspired. The Alphabet was used to translate the Kinderhook plates and was cited in Smith's letters as evidence of his translation abilities. It was kept in his office and he apparently considered publishing it in 1843. Given the Alphabet's content, it's difficult to imagine how it could have been derived save through revelation.
4) Visitors were specifically told that the deciphered portion was among the mounted fragments rather than on the missing portion of the scroll. According to an 1840 visitor to Nauvoo, Joseph Smith showed him some papyrus fragments that had been mounted under glass frames. “There, said [the prophet], pointing to a particular character, that is the signature of the patriarch Abraham.” The anonymous visitor’s report is confirmed by Josiah Quincy, who during a visit to Nauvoo was shown some papyrus fragments preserved under glass that the prophet said contained “the handwriting of Abraham.” Henry Caswall, too, was told that “a number of glazed slides, like picture frames … contained the writings of Abraham.” But the real clincher is a Quaker report that says Lucy Mack Smith “produced a black looking roll … part of which the prophet had unrolled and read; and she had pasted the deciphered sheets on the leaves of a book which she showed us.” Here the “deciphered” portion is explicitly said to be among the mounted fragments. This confirms the manuscript evidence pointing to pJS XI as the source of the translated text.
You also ignore scroll length accounts simply because it's convenient to make them as small as possible to ignore any possibility other than the Church is a fraud.
Please provide the text of the accounts I've ignored. As for my motives, you've got me wrong. Please consider the advice of Elder Orson F. Whitney, that we exhibit "less acrimony in public discussion, more charity with respect to what moves each man to do what he does do, and not to charge dishonesty and corruption until you have a real reason for doing so. . . I want to be charitable to my brethren and sisters and to my fellow-men. I don't want to live in my imagination as to their motives and acts. I don't want to think my neighbor a worse man than he is; I want to be just to him, and I desire, also, to be merciful." I beg you, Josh, not to think me a worse man than I am simply because you disagree with the conclusions I've drawn.