The original manuscript is not extant for the beginning of verse 36. There is a fairly large portion of text missing between extant portions of 𝓞 (namely, “desire to murmur & if it is not”). There is room for the ampersand in the lacuna, although the fit is better if the ampersand was lacking in 𝓞, at least initially.
When copying to the printer’s manuscript, Oliver Cowdery initially wrote the if-clause at the beginning of verse 36 without an ampersand. Oliver soon inserted an ampersand supralinearly— and without any change in ink flow, which suggests a virtually immediate correction. Most likely 𝓞 had the ampersand. There doesn’t seem to be much motivation here for Oliver deciding on his own to add an and to the text.
Summary: Retain in Alma 58:36 Oliver Cowdery’s virtually immediate correction in 𝓟, the and at the beginning of the verse (“and if it is not so”).