The 1906 LDS large-print edition changed the subjunctive were to the indicative was. In 1907 this change was adopted into the stereotyped plates for the third printing of the 1905 Chicago edition; subsequent LDS editions have maintained this emendation to was. A similar use of the subjunctive were is found earlier in the text, and this example of subjunctive were has not been edited to the indicative was:
For modern speakers of English, these two examples (in Mosiah 1:4 and Alma 8:31) are not contrary to fact, so was is expected.
The critical text will maintain the occurrences of the subjunctive were wherever they occurred in the original text. For similar examples involving “it were possible”, see the discussion regarding “if it were possible” under Mosiah 29:13 and “inasmuch as it were possible” under Alma 1:32. Also see the general discussion under mood in volume 3.
Summary: Restore the original use of the subjunctive were in Alma 8:31 (“neither were it possible that …”); similar usage is found in Mosiah 1:4 (“for it were not possible that …”).