Keep in mind that whatever HQ you take won't be able to come onto the board with Al'rahem. So the HQ that you take should be one that provides the most benefit to your army. Considering the Lord Commissar only provides benefits Imperial Guard units, I'd go with the Primaris instead.
Yup, that sounds about right. The only faction who are a bigger bunch of backstabbers than the Imperium are the Eldar (and this is debatable as the Eldar only stab OTHER factions in the back).