I don't think we see it
that differently. I was just pointing out that Mary, in the midst of having an ongoing affair, ALSO lied and violated Francis' trust in a pretty horrific way. There's just a certain amount of hypocrisy in her judging Darnley for doing the same thing.
I said this in another thread, but I do completely understand Mary's initial reaction. She had just discovered the betrayal. I know how hurt and disappointed she is, for all the reasons you just stated. My heart breaks for her. I just think she was a little rash to declare that she will never forgive him and they will never be happy. Then again...that was after he had declared that she will never love him. Ugh, what a mess.
Obviously I wouldn't be ok with my fiance sleeping with his ex, and I don't expect Mary to be. But there is a distinction between being engaged and being married. Breaking marriage vows and their promise of faithfulness before God is simply a whole other level of betrayal.
You're right, Darnley will be given a second chance, one way or another - "for the good of Scotland and the promise of England." One or both of them is going to have give in - even if they don't forgive.