I would just like to point out as an aside that the sound of stainless steel scraping on stainless steel gives me the jumping screaming wigglies. (Some restaurants serve their ice cream in stainless steel dishes with a stainless steel spoon and I have to return it and ask for a ceramic bowl.)
Just reading this thread is continually giving me the heebie jeebies.
Dump some lighter fluid in the pressure cooker under the stuck pan. Use a model rocket igniter to ignite it from a distance. I've seen people do this to reset the bead on a tire that's been patched. Also heard about people being killed this way. Don't stand too close.
I have been able to loosen shafts when stuck inside a cylinder by using two hammers, hitting the cylinder at diametrically opposite sides of the cylinder (e.g. stuck bicycle pedal cranks). This will cause the outer cylinder to flex, minutely and briefly, into an elliptical shape. The distortion tends to break the adhesion between the inner and outer surfaces which were stuck together.
I have even used it successfully with a glass bowl stuck inside another glass bowl - hitting with two wooden rolling pins! The minute distortion was enough to release the jammed article. It was in a coffee shop / kitchen supplies shop and this flash of brilliance earned me a free cup of coffee!
Just try giving it that bash / hard tap with the two hammers (move all round the periphery, doing the same thing - there may be just one or two stuck points). I could almost guarantee a result - as long as you are careful not to distort things permanently!
And, topologically, most of the food never actually gets into your body - it just passes through the hole running through the middle.
GUYS, THE FREEZING METHOD WORKED!!! (but only to a certain extent)
as was suggested, I filled the empty lower space with water and froze for many days and the ice actually expanded and pushed the pan a bit. OK, it didn't push the stuck lid out completely, but it was now possible to pass in a plier and WITH MUCH STRENGTH, pull the stuck pan out - Sophiecentaur, you're right - there were only 2 opposite points at which it was maximum stuck.
But it's finally out without having to use any destructive methods.
Thanks so much everyone for your help and suggestions.
BTW, the pudding looked good but was now all soggy and mushy because of the water I had filled in. And Dave, you'd better show me the "belly button" on the pot sometime.