Plumbing help for a dumb a$$

Guys

I've been renovating and changing out things since I bought the house 10 years ago. Haven't found many things I couldn't do. I think I might have screwed the pooch on this one - unless one of you can help.

I'm changing the faucets in a Jack and Jill bath. The fixtures and goose neck could be 50/60 years old. While under the sink, I noticed the bottom of the goose neck was corroded and spongy so I decided to replace it with PVC. The dead arm of the old drain - the part that goes into the galvanized pipe into the wall was seized. I took a screwdriver and drove it between the pipe and drain pipe. Took awhile but it finally broke lose and I finished the project.

Now bolstered with confidence, I started on the second one. Got to the drain arm inside the galvanized pipe and as suspected it was seized too. No worries - I started chipping away. Now I have a crumpled ragged drain pipe inside the galvanized one.
Any ideas on how to get the drain pipe out?

One option is to try and break the galvanized nipple lose - not sure I can do that and if I can, not sure I could blindly get it restarted since it is in the wall. The opening through the vanity into the wall has little or no play around the pipe and the vanity is built in. Hope I've give enough information where you understand the dilemma I've created.

Any advise and counsel is appreciated.