Here the printer’s manuscript lacks a finite verb form in the subordinate clause “when the fullness of my gospel be preached unto them”. One could interpret the be as a subjunctive verb form (in distinction to the indicative form “when the fullness of my gospel is preached unto them”). On the other hand, the 1830 edition has the modal shall for this subordinate clause (“when the fullness of my gospel shall be preached unto them”).
The reading of the printer’s manuscript seems to be in error here. The original manuscript probably had the shall and scribe 2 accidentally dropped it when copying into the printer’s manuscript. This analysis is consistent with the general tendency for scribes to accidentally omit small words, especially function words. Nonetheless, there is no specific evidence that scribe 2 of 𝓟 ever omitted modal verbs or that the 1830 compositor added them. Usage elsewhere in the text, however, argues that the original text here had the modal. There are over six hundred when- clauses in the Book of Mormon text, and not one of them is in the subjunctive; all are in the indicative (excluding, of course, the one case here in 𝓟 for 3 Nephi 20:30). More specifically, in the original text there are at least 19 instances of “when X shall be ” but none of “when X be ”. Thus internal evidence argues that the original text here in 3 Nephi 20:30 read “when the fullness of my gospel shall be preached unto them”. The critical text will assume that the subjunctive reading in 𝓟 for 3 Nephi 20:30 is an error.
Summary: In 3 Nephi 20:30 it appears that the shall of the 1830 edition was the reading of the original manuscript but was accidentally dropped by scribe 2 of 𝓟 when he copied the text from 𝓞 into 𝓟; elsewhere in the text there are no examples of subjunctive verb forms in subordinate clauses headed by when.