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.
Take care, guys and once again, Thanks so much.