The 1905 LDS edition switched the word order from the inverted will I to the noninverted I will, yet elsewhere in the Book of Mormon, when preceded by then, there are only occurrences of “then will I” (13 of them, of which seven are “and then will I”), none of “then I will”. And only one of these 13 other examples shows any variation in the word order:
The 1905 change in Mosiah 26:27 was probably unintended, as was the change in the 1852 LDS edition for 3 Nephi 16:11. For discussion of nearby examples of this kind of mix-up in word order, see Mosiah 11:6 (“yea and thus were they supported in their laziness”) and Mosiah 20:15 (“therefore in my anger I did cause my people to come up to war”). Also see the nearby example in Mosiah 26:28 (“him I will not receive at the last day”).
The passage here in Mosiah 26:27 parallels a well-known biblical passage in the King James Bible:
We note here the same order (“and then will I”), but also that the Book of Mormon verb is confess instead of the King James profess. Despite the orthographic and semantic similarity of these two words, confess is probably the intended word here in the Book of Mormon. Elsewhere in the text, confess refers to a sincere expression, most frequently an expression of guilt (14 times), but frequently as an honest admission of belief or knowledge (6 times). On the other hand, in the Book of Mormon, there are six references to profess, of which five refer to a false or undeserved claim:
There is only one example in the Book of Mormon where profess refers to a sincere expression— and that is in 3 Nephi 14:23, which agrees word for word with the King James text in Matthew 7:23! Thus the use of confess rather than profess in Mosiah 26:27 is normal for Book of Mormon usage. In addition, the Oxford English Dictionary lists under the verb confess the general meaning ‘to declare or disclose; to acknowledge, own, or admit’ (see definition 1). The verb profess has a very similar definition in the OED: ‘to declare openly, announce, affirm; to avow, acknowledge, confess’ (see definition 2). Despite their similarity in meaning, there is no specific evidence of these two words having ever been mixed up in the transmission of the Book of Mormon text, in either the manuscripts or the editions.
Summary: Restore the original word order in Mosiah 26:27 (“and then will I confess unto them”), which is the expected order in the Book of Mormon as well as the order found in Matthew 7:23; maintain confess in this passage since it is consistent with Book of Mormon usage, even though the verb in the related Matthew 7:23 is profess rather than confess.