It's possible that this was OM's last desperate attempt to keep her. If this is the first time you've seen hickies then he may have been trying to sabotage the reconciliation.
That's not to excuse her behavior, just going along with your feeling that it is "almost" over. If you are willing to forgive her for the affair then you also need to be prepared for some separation pains.
Definitely make sure W knows that seeing OM is a deal-breaker when it comes to her living with you. No "just friends" or "just lunch" or "just talking."
