Why does no one seem worried about how Robin will react when and if he finds out Regina was responsible for the supposed death of Marion?

You mean he really forgave Regina that quickly?

Possibly, but what she did was in the past and they're currently in the present/future, and like Marion said he did mover on.

Wow talk about honor! I amor Regina but not gonna lie. I wouldn't be so forgiving that quickly if the person I was in amor with was responsible for the supposed death of my anterior love.

That would be true.