I think Leena might have a contract of 2 years.. she entered the show in Oct 2014 or so.. She may have to stay until Oct 2016... so may be that's when she is exposed.
For the kind of crimes Tannu has done, Abhi won't have emotions or considerations for her after she is exposed. She is not like Aalu, who is Abhi's sister.. and Abhi-Aalu did share a bond unlike TaBhi.
So this is what might be driving the drag.
But then, Leena/Tannu has been a part of the successful show so far, so they may create more scenarios for her to affect AbhiGya after exposure too.