Here in the printer’s manuscript, Oliver Cowdery initially omitted the and. Later, probably when he proofed 𝓟 against 𝓞, Oliver supralinearly inserted an ampersand with slightly heavier ink flow. The connective and seems necessary here, given the connective style of the Book of Mormon text. The critical text will maintain the and here at the beginning of verse 31.
Summary: Maintain in Helaman 11:31 the and at the beginning of the sentence (thus “and they were again obliged to return out of the wilderness”); the inserted and in 𝓟 appears to be the result of proofing 𝓟 against 𝓞 and not editing.