The Book of Mormon text uses both insomuch and inasmuch. Here in 2 Nephi 1:20, Oliver Cowdery initially wrote insomuch in 𝓞, then immediately corrected the word to inasmuch. Undoubtedly, inasmuch is correct since it is repeated in the second, negative half of the promise (“but inasmuch as ye will not keep his commandments / ye shall be cut off from his presence”). Elsewhere in the text, whenever the Lord’s promise to Lehi (and also to Nephi and to others later on) is quoted, inasmuch is always the connector (18 times), never insomuch:
We should also note that in five verses we have a pair of inasmuch-clauses, and in each case, just as with 2 Nephi 1:20, the second clause is a negated version of the first clause: namely, 2 Nephi 4:4, Alma 9:13, Alma 36:30, Alma 38:1, and Alma 50:20. And for each pair of clauses, we always get inasmuch. Thus the initial appearance in 𝓞 of insomuch in 2 Nephi 1:20 is anomalous.
The last example in the above list actually shows textual variation between inasmuch and insomuch:
In this case, internal evidence suggests that inasmuch is the correct reading and that insomuch was accidentally introduced by the 1830 typesetter (see the discussion under 3 Nephi 5:22). There is another example that shows a typesetter (here the one for the 1874 RLDS edition) accidentally replacing inasmuch with insomuch:
Finally, there is one additional case where insomuch could be an error for inasmuch. For discussion, see 2 Nephi 9:15.
Summary: Maintain the use of inasmuch found in the Lord’s promise to Lehi, here in 2 Nephi 1:20 and numerous other places in the text.