Update: Well, after almost a week of trying different tools (wrenches, etc) and using muscles I am still unable to unscrew the galvanized steel pipe that's rusted to the cast iron drain.
I was told by a friend that one way I could try to remove the pipe now (as a last resort) was to saw it off as close to the drain as I could and then use the metal blade on the saw to cut the remaining piece with the threads inside the pipe.
He said to stick the saw gently into the drainpipe where the threads are and cut at positions 12, 3, 6 and 9 (on a clock) and then try and scrape or remove the last pieces of steel pipe.
Is this a "valid" way to finally get the pipe out, or is there anything else I could try? I'm in desperate need of ideas. THANKS!