Regarding the legality of Jagat-Gauri marriage - Gauri's entire arrogance and attitude derives from the fact that "she is the legal wife of Jagat". This is the driving factor in her shrill ear-piercing harangues, her insensitivity to others, rudeness to the village, and downright abusive behavior towards the family and esp. Anandi. Once she realizes that even that legality is denied to her and she is here and lording it over all and sundry only on Anandi's sufferance, that would be a fitting slap on her face.
Also, she got the sanction of her parents and Bade Papa only on the grounds of legality of her marriage, by disclosing this fact, even the family support that she has becomes questionable. Her parents and esp. her mentor would not support her, in this act.
True she can go in for a live-in relationship, but she'll always live in fear of exposure, and without social and legal sanction, not just for herself but also her children if she decides to have any.