It is not possible to compare these two. They are poles apart. Leaving and living with other man doesn't bring them at the same base. There is no relationship b/w Ishita and Mani except being friends and sense of responsibility and gratitude for saving her and giving new life.
Shagun wanted more from her life, while Ishita wanted nothing from her life; infact she wanted to leave her life. Raman asked her to leave in anger, she left the place and thought of ending up her life. To much surprise Mani came out of nowhere and saved her losing his legs.
To be true, if someone will ask me to leave and never come, I would also never return back to see if he really hate or was that just out of anger. Some things subside into your heart for ever. And even if Ishita wanted to come after she was saved, was it good to leave Mani who became handicapped in order to save her. That time Mani niece/nephew weren't too old to take care of themselves as well as Mani. Ishita felt it right helping her friend who almost became handicapped in saving her. There might be myriad of emotions running in Ishita's heart and mind. She herself wasn't sure; if she would be welcomed warmly by Bhallas specially Raman.
Apart from this, Shagun after leaving Raman also; never let Raman live in peace. She with Ashok planned lot against Raman and Ashok always showed off his possession to Shagun time and again in parties. I would have really accepted Shagun betrayal little easily if she would have never poked into Raman's life later. But she had a motive of ruining him time and again using Adi/Ruhi etc.
One more thing if Ishita now accepts the proposal of Mani, it's not that she is betraying her marriage. It's already 7 years and the marriage is null and void (according to Law). So, we shouldn't question even that now.
I can never agree Ishita=Shagun
Ishita was forced to leave while Shagun choose to Leave.
P.S. Sorry to hurt, but this is what I feel.