One thing i don't get at all is why Megha behaves differently with Mohan.
Mohan behaves with Megha like he would have with anyone else(till now). I don't think he has gone out of his way and did anything for her. Helping to find Nanhi is a very human thing. To add that Nanhi is his friend. So he choosing to help Megha is no big deal.
He likes to irritate Renu with his "ill-mannered talks"(dont know what to call it). He would have behaved the same with Megha too, if the guilt of Ad-fiasco was not on him. Even now he forgets his guilt and that Megha won't appreciate or even react the way he would like, he does talk his usual style out of habit. One Incident is the Hanky scene, when mohan calls her to return the Hanky, irritated Megha says "kya hai" and he replies "Byaah hai". Megha is clearly not interested in such replies. Mohan bites his tongue and gets to the real business of returning Addu's Hanky.
When megha held his hand and dragged him, he was very casual about it and said "haath chodegi tho chalunga na". She was embarrassed but he took it very normally.
Megha is always shown very restrained with others. Even after Rastogi's proposal scene, she came and gave tablet to Sanjay. She calmed herself and tried to behave normal. But she always blows off on Mohan. Is it because she is thinking Mohan is bad based on her previous encounters? But how does that justify her behaviour with Mohan - unnecessarily shouting at him, not just taking away his credit but letting someone else take it too, finding new faults in him. The way she shouted him at after getting Nanhi back, seeing Megha's expression, I don't think she was expecting him to wait for her. She just used it against him to not say "Thank you" to him.
Next day morning after seeing Mohan, she is like – let me say a "Thank you" to him and end this Guilt. Then she couldn't do that. In the promo too in kite-flying competition, she is holding his participation in the contest against him! So many others will also participating against her family. It is a contest in which the whole mohalla is participating. Does she expect Mohan to back off to make Vyas family win? Why does she behave like this? What's going on in her mind?