I see two reasons behind this fake marriage drama:
1. Coz he trust Ishani, but to get Ritika behind the bars he need solid proof, which he can only gather from her by making her realax by this thought that now hez only her and Ishani is no more in his life. He said to Ishani the day of suicide that u sign only those papers which I will give u, kyonki ab tumpar wishwas nhi kar sakta- it hinted to me towards that as if he meant that now I will sort out this matter my way, but won't let u know, kyonki agar tumhe bataya to pata nhi fir konsi MAHANTA karne nikal pado. So now he is working his way.
2. Coz after Ritika's this much torture, that they both part their ways. Ishani spent 6 mnths of her life behind the bars for the crime what she had not done, and could have been even died if not saved by Shikhar at last moment. Then Falguni Maa's murder. Then tried to kill Ishani and stabbed her so badly. For this all, just simply sending her behind the bars is JUST NOT ENOUGH.
Ranveer is on a moto to reveal her truth plus avenge for what all losses and problem his family faced due to her in past 8 months(as per show calendar) and good he avenge her for what all torture we also have to bear in past 6 months due to her 😡