Here Oliver Cowdery initially omitted the helping verb will; somewhat later, probably when he proofed 𝓟 against 𝓞, he supralinearly inserted the word (the correcting will has a more even ink flow than does the original inline writing). It is possible, however, that the original text here read “that I will do” and that Oliver inserted the will before the subject I rather than after it. Theoretically, either word order is possible. There are no other examples in the Book of Mormon of this kind of conditional statement, but instances in the King James Bible show that either word order is possible:
The closest parallel to Alma 18:17 is John 14:13, which has the same word order, “that will I do”. The original text in Alma 18:17 most probably read as corrected in 𝓟: “whatsoever thou desirest which is right / that will I do”.
Summary: Accept in Alma 18:17 Oliver Cowdery’s corrected reading in 𝓟, with the will preceding the I (“whatsoever thou desirest which is right / that will I do”); similar usage is found in the King James Bible.