What you are saying is correct. Sakshi has the tape , proof against Arnab's murder. Sakshi rehamat khan killed her husband and step son(alive). It makes Sakshi helping Rajnath sweep his crimes. In return she got Goenka title.
Rajnath was in love with Raima. He insisted to befriend with Sakshi to get that tape. Why she married To Jatin, may be to show Sakshi that she is married. And to avoid doubting about Rajanth's affair with Raima.
Sakshi did finded Rajnath's affair earlier, but may it was not revealed at that time it was Raima. RajMa Might have been sucessful showing other girl as Raj's love interest. To avoid such situations Raima may planned to marry someone , so she might have trapped Jatin for that.
That tape is the proof how Sakshi Rehamat Khan become Sakshi Rajnath Goenka.
That tape and Raima's Deposition may get back dev out of jail. May be that is at the end of the serial.
Karan is back , and Basu is shown for the first time, Kangana's affair and Downfall of Basu and karan's friends ahead.