As discussed under 1 Nephi 11:7, Oliver Cowdery tended to accidentally drop the modal shall in the future perfect. In both of these passages here in 2 Nephi 26, Oliver corrected his initial error in 𝓟. In each case, shall is supralinearly inserted; in the first case (in verse 10), the ink flow for the shall is somewhat heavier, while there is no difference in ink flow for the second case (in verse 15). The first shall may have been inserted somewhat later (perhaps when Oliver proofed 𝓟 against 𝓞). Later the 1837 edition accidentally deleted the shall from the first example; this error has continued in the LDS text, but the RLDS text restored the shall in the 1908 edition, probably because the printer’s manuscript reads that way.
The original text allows both the perfect and the future perfect in describing future events. The critical text will maintain each instance of the future perfect, providing it is supported by the earliest textual sources.
Summary: Maintain the future perfect readings in 2 Nephi 26:10, 15 (“shall have passed away” and “shall have dwindled in unbelief”).