Bride swap can easily be done in bts Avni is already in mandap but she may leave inbetween for some reason and go to room that time Kalmoksh can easily make her unconcious and Kallu becomes bride.
In regards to the pregnany Kallu can put on pillow or protestic belly.
The person who kidnapped AS may blackmail Kalyani to marry Malhar they may be bad in the show but that kidnapper will receive all the love, appreciation and blessing from viewers for kalma marriage
Even if Avni stays after this kalma will be legally married so her scheming and plotting will be tolerable and we don't have to watch her shouting "mera pati" or clinging to Malhar like glue in name of vaada.
I don't see Avni exiting so soon without turning full fledged negative so either her fake pregnany will be revealed by Moksh and DP and baby is the main reason behind the marriage or bride swap may happen hamara AS kidnap jo huyi hai.
Bride swap is more possible in my opinion because Avni pregnancy getting exposed and then her easily letting Malhar marry Kalyani doesn't seem likely. Kallu will marry Malhar to save AS afterall she is our mahaan and self sacificing FL but this time her sacrifice will prove to be of some benefit to herself rather then the idiotic one to make Malhar marry Avni.