That coupler, however has caused me to invent an entire series of new curse words. I've now tried multiple size pickle forks which just resulted in stuck forks that took a small sledge to free. 2 foot wrecking bar just demonstrated how surprisingly much that bar could flex. Claw hammer. Sledges. Blackmail. Voodoo.
If you must remove the coupler you can try packing dry ice around it. It should create enough shrinkage to break the bond between the parts. Assuming the roll pin is completely out. Make sure your hands are well insulated.
Why are you removing the coupler anyway? If it is that hard to move I would just wipe out the old grease and put in some fresh and be done with it. Yes, I am lazy.