I am really trying very hard to understand the writers here. This post is not from a viewer who is obviously pissed. But from a neutral pov who just revisited the show to connect the lose strings.
So if we are going to start from the very beginning, it was very clear that both the brothers are going to fall for the same woman. Both Rishabh and Karan are shown as two completely contrasting personalities * Aag* (fire) and *paani* (water) who look at same situations with different approaches. However, one thing is similar between them. They both are verbally challenged. On one hand Rishabh couldnt say his feelings to preeta despite of umpteen opportunities he had after he realized he was in love with her. Whereas Karan never bothered going to that road with preeta initially despite obvious attraction.
However there are many scenarios in which the two brothers have talked about falling for the same woman and eventually they decided to change the woman when/if the situation arises. This could have been a very subtle but important link for the future tracks.
This karan Preeta wedding was not an impromptu decision by the writers. There's an episode where karan had said "Tum chaho na chaho preeta arora agar mujhe tumse shaadi karni hogi toh mai tumse shaadi karunga" Karan preeta forceful wedding was always the plan. Writers obviously left it on the fate of the show to decide with whom preeta is going to end up with. Karan is opposite preeta. That we all know. WHat we dont know yet is whether this opposite is in terms of angst or happily ever after.
There is no drop in karan's character. He was always a snobbish spoilt over pampered over grown kid who was living under rishabh's shadow and protection. He was never an evil person but what he did with preeta at the night of their wedding was nothing short of evilness.
Now with rishabh's declaration, (Of not supporting karan) , now is the time when probably writers will give karan to think about all his doings.. Even though he will definitely carry out this wedding until something major happens, this constant war between his heart and ego will go on.
But what's more important is, whether or not karan or loothras accept preeta as their bahu, RIshabh has accepted her as karan's wife. RIshabh has realised that despite karan being in constant denial, he is in love with her. And rishabh is a man of integrity and values. I don't think in any way, rishabh will marry preeta . Or will be in anyway associated with preeta romantically when karan is in love wit her.
It is going to be really difficult to gauge karan's mind right now but all other characters are pretty much clear. Specially Rishabh's.
Writers are playing safe with preeta. She probably hates Karan right now but is certainly not indifferent to him. He brings out the worst in her and vice versa. Had preeta been indifferent to Karan after what he did, then rishabh preeta or preeta and some new guy could have been an option.