The 1906 LDS large-print edition replaced do with to, giving “ye have gathered yourselves together and to marvel”. The source for this typo was undoubtedly the two occurrences of to marvel in the immediately following text: “yea and ye have great need to marvel / yea ye had ought to marvel”. No subsequent LDS edition followed this reading, “and to marvel”, since the 1906 edition was never used as a copytext.
This minor variant makes one wonder if the original text might have read “ye have gathered yourselves together to marvel” (that is, without the and ). Yet the semantics here suggests that “and do marvel” is correct since in this case the people wouldn’t have gathered together for the purpose of marveling. Instead, these people had seen Nephi praying with great sorrow and had therefore gathered together “that they might know the cause of so great mourning for the wickedness of the people”. It was his prayer that made them marvel. There are numerous examples in the text of people gathering together to do something (for example, to hear someone, to fight or go to battle, or to sing and dance). But these are specific acts where the individuals are acting as agents. Marveling, on the other hand, is a result.
The use of and with the auxiliary verb do is, of course, characteristic of the Book of Mormon text, as in the following examples involving people gathering together:
Thus the earliest reading here in Helaman 7:15 is perfectly acceptable and should not be emended.
Summary: Maintain in Helaman 7:15 the earliest reading, “and do marvel”; the context as well as usage elsewhere in the text supports this reading.