Here in the printer’s manuscript, Oliver Cowdery initially wrote “and Abinadi saith unto them”, but then sometime later he crossed out the ampersand he had written and supralinearly inserted but now (the correction was done with heavier ink flow). Undoubtedly, Oliver’s original and resulted from the consistent occurrence (up to this point) of and in the exchange between Abinadi and the priests:
In particular, here in verse 33, Oliver was probably influenced by the immediately preceding “and they answered and said”. The and that Oliver initially wrote at the beginning of verse 33 would have worked, so probably Oliver here corrected the text of 𝓟 to agree with 𝓞, no longer extant for the book of Mosiah. For a similar example of proofing against 𝓞, see the nearby correction of sent to cause send in verse 29, also done with heavier ink flow as here in verse 33. For the same reason, the critical text will maintain Oliver’s corrected reading but now in Mosiah 12:33.
There is also additional evidence in the manuscripts that Oliver Cowdery sometimes initially wrote and in place of but:
Notice that in each of these instances, there is at least one preceding and that could have served as the source for Oliver’s incorrect and; the same situation is found here in Mosiah 12:32–33.
Summary: Accept Oliver Cowdery’s corrected reading in Mosiah 12:33 (“but now Abinadi saith unto them”); this correction probably occurred while proofing 𝓟 against 𝓞.