My opinion;
General
I think it's someting cultural (that lays in the old times and traditions, to surive, protect the bloodline).
If somebody have a good name reputation, you can trust him and invest in a good relationship (think about trade, wedding relationships in India, China).
Even today, respect (family reputation, pride) is very important in some cultures.
About your first question
The people in rual areas are often poor. Their honour is their most value thing.
And I think, that the question is not, that they don't see injustice dear. They do see it, unfortunately they don't act in the most cases against it.
So, they see injustice, but they don't have the courage or the power for some reason to do something about it (to fight for insaaf). Or the same honor, they want to protect (or keep high) doens't allow them to improve their own condtions.
Second question
About Brij, he is a male and the heir of the Handa family.
In this storyline, the men have the power to take desicions in the family.
But I think, his family has spoiled him to much and give him too much freedom. If they had given him some 'tapper' in his childhood and teach him some respect for women, then he would act diffrent today to women.
I din't read the book, but it sounds nice. If it also availble in the English languge, I will read it ;)
Some people live still in the past, while the rest of the world do live in the present.
Their is miscommunication between elders and young people with other opnions about their lives, dreams.
To stop such atrocities, I think tradtions have to change and more education for everybody.
So everybody can understand each other more and respect their opinions and their right to chose for themselves (their job, bride ect).
To act different, you must first change your view, the way you think about things in your life.
Third question
I think Maan is right in his view. Your only have to look his side of the story. The poor fellow did thought that he bought land from Geet (from Dev as dealer) he has got the offical documents of the zameen.
(It was a mistake of geet's family to give Dev the papers and give geet's signature).
Ofcource, he will think that Geet doesn't want to give him the zameen back and wants more paisa.
And the time, that he found Geet, she was already free of the ropes.
He only found her locked in a room, so he couldn't see, that her live was in danger.
To prove her innocence to Maan, Geet must first find Dev and get a confession that he didn't deceive her only but also Maan.
If she can convice Maan of this, than he will help her.