Here 𝓞 is sufficiently extant to determine that there is an and in the text: “the Lamanites dearst not come into the heart of their lands & attact that great City Zarahemla” (original accidentals based on 𝓞, where extant). When Oliver Cowdery copied from 𝓞 into 𝓟, he originally wrote to in place of the original and: “the Lamanites dearst not come into the heart of their lands to attackt that great City Zarahemla” (original accidentals). Virtually immediately Oliver caught his error in 𝓟, crossed out the to, then overwrote it with an ampersand (there is no change in the level of ink flow for the correction). But the 1830 compositor ended up setting the secondary to, which has been retained in all the printed editions. Here in Helaman 1:18, the critical text will, of course, restore the and, the reading of the manuscripts.
There are a number of places in the text where the infinitival to and the conjunction and have been mixed up. In each case, the critical text will follow the earliest extant reading. For discussion, plus a list of examples, see under Mosiah 21:18.
Summary: Restore in Helaman 1:18 the original and in “the Lamanites durst not come into the heart of their lands and attackt that great city Zarahemla”; the conjunction and is the reading in 𝓞 and the corrected reading in 𝓟.