Here Oliver Cowdery supralinearly inserted strictly after did observe. The insertion is without change in ink flow and was probably virtually immediate. Undoubtedly, the original manuscript had strictly. One wonders, however, if Oliver inserted strictly in the right place. For instance, it could go after the verb keep—or even right before keep since so-called split infinitives occur elsewhere in the Book of Mormon text (as in Helaman 6:29, “to still carry on the work of darkness”).
There are three other occurrences of strictly in the text, and one of them places strictly immediately after the verb observe and before the infinitival to keep:
Thus Oliver Cowdery’s insertion of strictly after observe in Helaman 13:1 is probably correct.
Summary: Maintain in Helaman 13:1 the corrected reading in 𝓟, with strictly coming immediately after the verb observe (“while the Lamanites did observe strictly to keep the commandments of God”).