[RCF]

One possible outcome could be Nikitha stays with Jay with Tarun as bit on side or vice versa.
I hope that does not happen.
At this moment she has both men wrapped around her little finger.
Mithra's role (crucial ????) has been alluded to by writer in one of the comments.
We have totally forgotten about the crucial letter.
Too many twists and turns.

About the letter, 

Simple and easy would be to put it on Tharun and blame to make him a villain but I do not think that's the plan..
Anjali is another option which doesn't impact much in this triangle relationship but would prove to Nikita Jay is innocent...

My go to plan would be either Mithra through some one else maybe thorough Anjali planned to separate Tharun and Nikita to save her love...or Jay himself wrote it to make Nikita hate him and free her from the burden she was carrying at that time as he saw despite caring for him through fever she was struggling with duty and wife responsibilities and her pursuit of love. He might have put an act with Sharmi but he saves Nikita from himself there by putting the most selfless act to prove his love...

One of these should would suit well. 

~ RCF