For the first time I agree almost 75% with you.
Dr Mahajan's matter
I don't think it was fair. Kisi ke behaviour ko napasand karne mein( ya uss wajah se uss par gussa hone mein) aur usse galat kehne mein farq hota hain. She wasn't wrong on being angry with Karan because of his behaviour and expressing the same openly but when she said him to apologise, there she went wrong because she directly passed the judgement of Karan being wrong without listening the whole matter from both sides.
Love changes People
During a date or them just spending time with each other, on questioning about Balli's sweet or affectionate nature, he can reply that 'main tumhara boyfriend hu, uss haq se main sweetly ya thoda pyaar bhi nahi jata sakta, aur ab toh kisi ko hamare baare mein pata chal jane ka dar bhi nahi' (I guess Balli watched KarMi in the club and observe them like how close they stand, their expressions while talking to each other, again correct me if I am wrong, because spoiler mein itni clear nahi thi yeh baat) but again it all depends on how well Balli imitates Karan, like you said.