How can I remove the drain flange from the drain assembly?
Buy or make a close quarters hacksaw.  If it has a pistol style handle, turn it around so it's opposite the teeth.  Carefully cut a slit in the flange, then grab the flange with pliers just as in your second picture, crumple it in on itself and push it down through the hole.


While you're working on this cover the drain tube with a sandwich bag using a rubber band to hold it on, to prevent sewer gases entering the room.