Oliver Cowdery initially wrote the singular condition, which is what we expect in current English. However, almost immediately he corrected the text by inserting the plural s (with no change in the level of ink flow). Theoretically, Oliver could have made the change to the plural because two distinct conditions follow (“deliver up the king Noah … and deliver up their property”). But evidence from the rest of the Book of Mormon text shows that the text itself consistently prefers the plural conditions, even when there is only one condition listed. Consider, for instance, the original reading for the four instances where conditions is preceded by the preposition on. These four cases are semantically equivalent to the one case with under (here in Mosiah 19:15):
Note that for two cases (Alma 27:24 and Alma 54:11) there is only one condition, yet the text still uses the plural conditions.
Based on the earliest textual sources, there appears to have been no occurrences of the singular condition for the entire text, only conditions (14 times). Nonetheless, there has been a strong tendency in the history of the text to replace the unexpected plural with the singular—sometimes accidentally, sometimes intentionally. Besides the case of Mosiah 19:15, there are four other instances:
See each of these passages for discussion, especially the last one (which turns out to be more complicated than the others).
Thus the critical text will restore the consistent use of the plural conditions throughout the Book of Mormon text, including here in Mosiah 19:15: “under the conditions that they would deliver up the king Noah into the hands of the Lamanites and deliver up their property”.
Summary: Retain in Mosiah 19:15 the plural conditions, Oliver Cowdery’s corrected reading in 𝓟; the original Book of Mormon text consistently used the plural conditions, never the singular condition.