The 1892 RLDS edition dropped the conjunction and here at the beginning of this verse. This loss appears to have been a typo, although the use of and before both clauses in the sentence may have seemed strange (“and only a few years have passed away and they were a civil and a delightsome people”). The 1908 RLDS edition restored the original and to the RLDS text.
Elsewhere the text has six examples of sentences beginning with and only, as nearby in Ether 12:40: “and only a few have I written”. The critical text will maintain the earliest reading here in Moroni 9:12, with and preceding the only.
Summary: Maintain in Moroni 9:12 the occurrence of and before only, the reading of the earliest text.