I really feel they don't want Zaid to surface those feelings. It can't come up while he's devoted to Ayra. He confessed this now only because MJ asked him pointedly and Zaid believes as always he should be fair to her too. It's a very thin line they have written their hero on- he has wooed and won one and loved and admired the other and lost her once.
For me, as a viewer, if they don't do it very subtly, I'd say he's cheating on both. That's why I still don't think there was a place for FB till now.
There's always the question. If he loved her and moved on, why did he choose her again? It could have been any other girl, but her. That aspect of his character is very gray and the only way they can make us respect him is to keep us guessing, knowing the hero has locked it away and never means to open it.
They can make him say he has nothing for her now so it's a pure business deal to marry her...then he could have had that intense hatred track as well...and the second marriage and him assessing her again...then it's in a dangerous zone where he's checking her out while he's married to Ayra. We wouldn't feel the same purity in his feelings towards Ayra or MJ. Purity is not the word, perhaps...decency ...or... respect...and they wouldn't be able to develop it like him reading Mubashira like a second reading...may be his actions would have had more clarity, but it may not have looked respectable is what I feel.
Is this MJ edition 2 much different from the first edition? Not really. She has just mellowed her approach towards him...which actually she was trying even before she tried to force him to accept her. She had shown concern, tried to talk it out with him.and the last distress call to tell him whatever it is, they could talk it out before he took any extreme step ...it's the same MJ we are seeing now...just that the tenderness for him is her main trait now. She's able to take much more than she could before without keeping any tabs. She started giving, saw she enjoyed it more and she decided this is what she wanted.
What does this new revelation do to them now? Was it to clarify anything to the viewer as such? Not really. It was to clarify to her that even if she finds him reciprocating her feelings, there's always the permanence of what her rejection did to him. He's honestly telling her why he can't go beyond that and she can only hope this changes one day. That has to come from him...she can only do this much because he has clearly told her he will draw his limit at one point, no matter what his heart tells him.
This is a very important crossroads Mubashira is standing at. She can wait, hoping he accepts her whole heartedly or she can choose her path. Mubashira might not process the seriousness of what he said, right now...she's uneasy and sad about it, for sure. May be she will revisit this dialogue and examine how it translates to the future of their relationship. For me, this is the relevance of this dialogue at this point in the story...it was not to shock viewers into a revelation...though it served that as well...(even I was shocked she knew about it)
This is why I don't see much point in a backstory. What happened between them is not so relevant to me when I'm waiting to see what's going to happen to them.