I know I am answering this so late, meant to the other day but as usual I popped in and out within minutes...😃
So here goes, my theory is it is written by Arpita herself, from where ever she is. My impression of the letter was one written by someone, trying a last ditch effort to have a wrong righted. However, the sense I got was it was a prelim, just before the actual attack - almost testing the water...and/or trying to throw someone off balance.
So as Arpita is coming back as Sonam, she is ready to take revenge on those who caused her grief. So she knows RV won't have the goodness in him to do the decent thing, but she wants to see his reaction/test his reaction. So she sends in this garbled letter - deliberate to make him think she is distraught and unhappy...he reads, laughs and burns the letter. Arpi wherever, she is knows he has done nothing - maybe for her conscience sake she wrote that (giving him one last chance to tell the truth) - so she takes her blessing and sets out to punish.
My two cents.....😃😉