Óskar ániyë:

 What I meant with French possibly having preserved the "archaic Indo-
 European word" was not exactly that the IE form would have passed directly
 to French, but rather that French had somehow preferred 'ne', which was
 after all a negation in Latin (the original one, 'non' being the compacted
 composition 'ne oignom' AFAIK). I.e., that Gallo-Latin speakers had
 used 'ne' at all times to negate, instead of 'non'. But I considered the
 sound change explanation at least equally likely.

 In that case, the negative interjection (?) 'non' in French has that form
 because of its stress, am I right?

  Sounds like a possibility to me!