Hi Doc, I'm going to agree with you where you doubt Rajveer is illegitimate son of Harsh
The series story line never said that Harsh had another child directly, all we know is that Harsh left Anjana with Aditya and Arjun to live with another woman for some years but then came back. In the jungle episode track when Harsh & Arjun were trying to get evidence to help Aditya and Zoya, Harsh's first impression on meeting Rajveer was that he reminded him of someone but could not place him (I'm going by memory as I'd have to watch the episodes again to get the exact dialogue ). If Harsh had indeed raised Rajveer I doubt he would forget having another child no matter how long the absence or indeed what they looked like! Then Rajveer made a statement about use of the law & misuse of power. My guess is this and I am happy if I am wrong, is that somehow there was a miscarriage of justice involving Rajveer's father which led to his death and Harsh would have the lawyer or judge on the case at the time and somehow a miscarriage of justice applied. Harsh may then have been involved with Rajveer's mother or may not. Her wearing of a mangalsutra does not necessarily mean she married to Harsh, but that at the time the photo was taken she was married. She may well have died later, in any case it may well explain the intense hatred Rajveer has for the Hooda family and what better way to seek revenge then to target Harsh through Aditya. Evil Nani keeps making reference to Aditya as being the eldest son of Hooda household. If Rajveer was Harsh's son legitimate or otherwise he would be the eldest son. Anyway I might be totally off track on all of this, but I look forward to seeing what unfolds.