My thoughts on the matter, which I posted as a comment, but which I'd like to post here as well, since they are thoughts on this week's parsha:
My first pshat-oriented instinct is that the question doesn't start, because of course וילך doesn't mean that Moshe went anywhere, and that it means something along the lines of "continue." And that any modern pashtan worth his salt would tell you that. And indeed, Baruch Shekivanti, Shadal says just that. See here:In terms of the LXX, isn't this obvious, that an apparent mistranslation would be based on a variant text? That the Dead Sea Squirrels also have this variant text is not so surprising either. But the Masoretic Text is no slouch either.Many LXX variants are based on obvious typos. For example, Moshe says "I did not take any desirous thing from them," because they had חמוד with a daled rather than חמור. The question then arises as to which is the "better " of the two texts, LXX + DSS or MT, in the particular case. In this particular instance, I think that the principle of lectio difficilior might argue in favor of the MT, because it is something that seems to be difficult and would prompt reinterpretation / "correcting", even though on deeper analysis it actually works with little problem.