Here the 1840 edition replaced the demonstrative adjective this with the definite article the. This change could well be a typo rather than the result of conscious editing. The previous use in this verse of the before five (“the five which was sent to inquire concerning the chief judge”) may have prompted the change from this five to the five in the 1840 edition. In 1908 the original this was restored to the RLDS text.
Grammatically, one could object to the use of the singular this in “concerning this five” rather than the plural these (“concerning these five”). Such a factor may have led the editors for the 1920 LDS edition to adopt the 1840 reading with the the. This change was marked in the 1920 committee copy, which means that the change was a conscious one. In any event, the 1981 LDS edition restored the original this to the LDS text. The use of this in this context is actually appropriate since the people are trying to identify which five were sent and which five were alleged to have killed the chief judge. In such a discourse, one can refer to “this five” or “that five” as well as to “these five” or “those five”.
Summary: Maintain in Helaman 9:12 the original reading “this five which ye say ye have sent” (also the current reading in both the LDS and RLDS editions).