Difference between Shreya & RG : 
Though RG is a hardcore criminal but it is also true that if he feels that his goal is achieved then he does not look behind as for last 8 yrs he was never do anything to Kabir or Kashyaps. Now, if Kabir & Ananya personally meets him & promises him that they will never try to pursue SK case as after Ronnie's death they cannot loose any of their loved ones and they have realised how powerful he is then RG will never try to harm them. They can keep this matter close for some time. just to fool RG. If the justice can wait for 8 years then why not some more months. 
But, Shreya's only aim is to kill Ananya to get Kabir back. She only feels one thing is important in her life to see Ananya is dead. She understands that Ananya's love is so deep that it is impossible to separate Kaya. So, now, she feels that if she kills Ananya , Kabir will be her. She will go to any extent to achieve her target. So, Shreya is now became dangerous for Ananya. Shreya took some medicine we have seen in last episode. Her psychological disorder is now can be seen. Manav has seen it , today in bits ( it is different issue that Ankita could not enact it properly) . So, I don't know how long it will take to break her down totally because till then Ananya is very very unsafe & her main support has gone.
So, I really feel RG can be handled for now & then they can again attack him with full proof plan but Shreya is unstoppable as she is a psycho.