The original manuscript is not extant here. The printer’s manuscript has an ampersand before thus, but the 1830 edition lacks the and. Chances are greater that the 1830 typesetter accidentally missed the and (written as an ampersand) than Oliver Cowdery accidentally added the ampersand. When we consider errors of transmission involving and thus, we find evidence for the loss of and from and thus but no examples of adding and before thus. But it should be noted that all of the examples are found in printed editions of the Book of Mormon:
The connective and thus occurs several hundred times in the Book of Mormon text. But many sentences begin with simply thus, although not as frequently as with and thus (for instance, in 1 Nephi there are 13 instances of and thus at the beginning of sentences but only 5 of thus). Either reading is possible here in Helaman 16:14, so there would have been little conscious motivation for either Oliver Cowdery to add an extra and or for the 1830 typesetter to delete the and. Given the tendency to accidentally omit and before thus in the printed editions, the odds are greater that here in Helaman 16:14 the 1830 typesetter is the one responsible for the variation; in other words, he seems to have accidentally omitted the and in his typesetting.
Summary: Restore in Helaman 16:14 the and before thus, the reading in 𝓟; the chances are greater that the 1830 typesetter accidentally omitted the and than Oliver Cowdery accidentally added it when he copied from 𝓞 into 𝓟.