text: If it were me, I'd contact fasteignaskrá and ask them to confirm, after presenting the ground truth findings. The buildings in Vífilsstaðir are in a ‚typical legacy setting‘ which might mean the building numbers were (probably) determined before there were any rules on how they should be placed along the street. Maybe the answer is to keep both, if fasteignaskrá confirms the official truth.
  
  (I do like your effort to resolve the notes I left all over the place so many years ago.)