When you've tried those ideas and failed, go to a hardware store and get a properly sized screw extractor (a.k.a. easy-out). Grab the easy-out with a pair of vise-grips, turn counter-clockwise, and the nipple will come out.
I have used this approach successfully after solvents, etc didn't help. And after futzing around with a nipple wrench, the flats on the nipple are usually boogered, so the wrench just slips off.
Some nipples, maybe Ampco, have deeper flats, which helps.
You may have to first run a drill bit through the nipple to enlarge the bottom hole to prevent the easy-out from bottoming out before it engages the nipple i.d.
Let us know how you get the nipple out.