I guess its not about not being able to see flawed FL but its more about not being able to see stupid FL or ML for that purpose.
If it would have been just shouting scene then nobody would have pissed off at katha this much.
She was confused, there was chaos everywhere and viaan was being too imposing at that time, although he had his own reasons and fears for behaving like that, the shouting at that time seems plausible real life like scenario.
The problem started with --
sending viaan to jail
by not coming to meet him,
by not maintaining contact with him,
by not being able to manage even one right occasion with aarav to explain things to him,
By taking him to doctor and not revealing the actual cause of his problem,
By bowing down in front of his stupid demands. For god sake he is just a child, an emotionally disturbed child, one can't take life's important decisions because their depressed child threw a tantrum.
By entering into a relationship where she is not committed fully and fooling whole family of doctor.
By not confronting viaan directly like in imagination scene.
Your problems are exactly my problems right now and these are valid problems. But, in the post you quoted, I am talking about that episode where Katha shouted on Viaan. When that episode was aired. Many tweets, plenty of tweets I mean really plenty of them were like the tweets right now. Right now, anger somewhat makes sense but, not at that time. As you said shouting was plausible then. I was talking about that time. Seeing this sentiment at that time made me lose faith of giving any rational argument.
Now, the problems in @bold, may or may not persist. Slowly but surely writers are tackling one problem after other through the flashbacks and slowly swaying the audience's perspective. However, I would have liked one long fully fleshed flashbacks instead of these short snippets. They will not justify everything Katha is doing right now but, may provide a convincing reason for her current decision.
They just need one scene to show that Katha explained everything to Aarav but still, it made no impact because the aftermath was simply too severe. Viaan got jailed, Kailash got shot, Later Kailash died, Katha probably left Grewals, They became lonely again. How can you fool child like Aarav regarding this. What explanation would you give to Aarav? But, they should first show something like this.
We indirectly know that any amount of explanation wouldn't have worked for Grewals. So, all arguments that Katha should have said this said that are automactically shut off. Similarly a flashback scene like this will shut off all Katha should have explained like this and that to aarav comments. Till such scene is shown, feel free to criticize because current track deserves criticism and amendments.
At that time I saw some comments why Katha didn't tell Viaan about Maaya Mausi blackmailing her. Like, isn't it obvious what happened next. Maya Mausi was simply unstoppable. Would you still say that ? Viaan knew Maaya Mausi blackmailed Katha what was he able to do? Did it solve any problem? He was unable to do anything.
In short, the comment you quoted, I was talking about the situation of fandom some weeks ago and feel free to criticize right now because there are things to criticize till the writers make the amends.