Robin should find out how Regina had Marian executed as an example of what happens to those that helped Snow. Marian is alive now only because Emma intervened.
I agree. That’s why I can’t buy the idea that Marian will somehow die and Robin and Regina will then pick up where they left off. It’s inconceivable that Marian won’t tell Robin what happened to her and once he discovers that Regina intended to kill Marian (and presumably did in the original timeline), he may forgive Regina, but he’s certainly not going to be eager for a romance with her, even if Marian should be out of the picture. Well, maybe if Regina risked her own life trying to save Marian and/or Roland from some menace, but even then it would be a difficult relationship.