In his editing of this passage for the 1837 edition, Joseph Smith grammatically emended the subjunctive were to the indicative was. Two other instances of subjunctive were have been left unchanged in this chapter; in both cases, we expect the indicative was in modern English:
But later, in Ether 15:14, Joseph made the same change of were to was in “and that they might receive all the strength which it were possible that they could receive”.
The Book of Mormon text strongly favors the subjunctive were in the phrase “it was/were possible”. For instance, for the conditional clause “if it was/were possible”, the original Book of Mormon text has ten instances with were but none with was. And for the ten remaining cases of “it was/were possible”, all but one take were in the original text (the one case taking was is in Mosiah 21:18: “now the people of Limhi kept together in a body as much as it was possible”). In other words, the use of were here in Mormon 2:16 is expected from a textual point of view. The critical text will restore the original were here.
Summary: Restore the original subjunctive were in Mormon 2:16 (“before it were possible to stop them in their retreat”); such usage is favored in the original text of the Book of Mormon.