Here in the printer’s manuscript, Oliver Cowdery had difficulty spelling the word ceased. He first started out with ce, crossed that out, and wrote inline the word as seaced. Then he crossed out the seaced as well as the original ce (a second time) and supralinearly inserted the correct ceased. In his supralinear correction, he also wrote the gerundive striving. The following phrase with them is written inline, which suggests the possibility (rather unlikely) that the original manuscript here actually read “lest the spirit of the Lord hath ceased with them”—that is, without the word striving. Since there is nothing particularly wrong with that reading, Oliver’s decision to supply striving does not appear to be an attempt to emend the text; instead, 𝓞 very likely had striving. Elsewhere the text always has the verb strive in expressions that refer to the Spirit of the Lord ceasing to influence people:
Of course, one could argue that the two nearby instances of ceased striving (in Ether 15:19 and Moroni 8:28) led Oliver Cowdery to add the striving here in Moroni 9:4. Ultimately, the safest decision here is to follow the corrected reading in 𝓟 since it appears to be virtually immediate. The original motivation for the correction was the spelling of ceased, not the occurrence of striving.
Summary: Maintain striving in Moroni 9:4 even though it was supralinearly inserted in 𝓟 along with ceased when Oliver Cowdery corrected the spelling for ceased; usage elsewhere supports the expression “to cease striving”; if 𝓞 had read without striving, there would have been no strong motivation for Oliver to have added it when he copied the text into 𝓟, although there is a small possibility that he was prompted by two nearby instances of ceased striving.