In verse 10, the 1920 LDS edition changed in him to in whom, probably because the editors wanted to make sure that the existential clause “there is shadow of changing” was conjoined with the preceding relative clause (originally “which doth vary” but later edited by Joseph Smith to “who doth vary”)—that is, “a god … in whom there is shadow of changing”. Of course, there really isn’t much of a problem understanding the original text here, especially since the beginning of the following main clause is marked by the adverbial connective then (“then have ye imagined up unto yourselves a god which is not a God of miracles”).
It is possible that the him found in the earliest textual sources is actually an error for whom. The original manuscript undoubtedly had the him (since both 𝓟 and the 1830 edition here read identically), but Oliver Cowdery, the presumed scribe in 𝓞 for this part of the text, might have misheard Joseph Smith’s whom as him (which are acoustically similar and differ only in the vowel). There could have also been some influence from the occurrence of in him at the end of the previous verse: “and in him there is no variableness neither shadow of changing” (Mormon 9:9).
The derived construction “in whom there is …” occurs nowhere else in the Book of Mormon. It is found once in the King James Bible and also once in the Doctrine and Covenants (in a revelation dating from 4 February 1831):
The example from Psalm 146:3 shows that the formal sounding “in whom there is …” is allowed in the biblical style. Note that the example from the Doctrine and Covenants is based on the King James language in John 1:47, which reads without the there: “behold an Israelite indeed in whom is no guile”.
Another factor to consider here is the parallel language between Mormon 9:9 and James 1:17 in the King James Bible:
Note that here the Book of Mormon him corresponds to the King James whom. Also, the existential there is left unstated in James 1:17 (just as it is left unstated in John 1:47). The parallel language could lead one to argue that even the him in Mormon 9:9 was originally whom. Still, it seems rather unlikely that both instances of Book of Mormon him, in verses 9 and 10, are errors for whom.
One may also wonder whether the lack of the indefinite article before shadow is correct in verse 10. Originally, scribe 2 of 𝓟 wrote “& in him there is a shadow of changeing”, probably because English speakers expect the indefinite article after there is in such a construction. The a was crossed out, either by scribe 2 or later by Oliver Cowdery when he proofed 𝓟 against 𝓞 (there is no apparent change in the level of ink flow for the crossout). Undoubtedly there was no a in 𝓞, which explains why the 1830 edition, an independent copy of 𝓞, reads “and in him there is shadow of changing”. Nonetheless, it is quite possible that the a was missing in 𝓞 simply because Oliver Cowdery, the presumed scribe, accidentally omitted it. On the other hand, the lack of the article a may be intentional since there is no a in the preceding verse (“and in him there is no variableness neither shadow of changing”); theoretically, the preceding verse could have read “and in him there is no variableness neither a shadow of changing”. But note that the parallel language in James 1:17 of the King James Bible lacks the a in “and in whom is no variableness neither shadow of turning”.
More importantly, we can find evidence on for the expression “there is shadow of X”, although all the examples postdate the Book of Mormon:
These examples further show that the lack of a in Mormon 9:10 is possible. Consequently, the critical text will allow the earliest reading, “and in him there is shadow of changing” (that is, without the a) in Mormon 9:10. Similarly, verse 9 will be maintained without an a before shadow (“and in him there is no variableness neither shadow of changing”).
Summary: Restore in Mormon 9:10 the original him in place of the secondary whom; also maintain the lack of a before shadow, the 1830 reading and the corrected reading in 𝓟 (“and in him there is shadow of changing”); the preceding verse also has him and lacks a determiner for shadow (“and in him there is no variableness neither shadow of changing”).