Congrats on being able to twist it! Part of the problem is all the particulates in the tube. They do act like sandpaper and prevent extraction. I don't have near the experience as say wrk101 but the post I removed was stuck badly and was a challenge to remove after getting it to rotate. My approach was to place my stocking feet on the DT and chain stays while pulling and rotating the post with a saddle, I didn't care about. It was very difficult and became easier as more of the post was exposed.

Once it moves out, check for gouges as mentioned by [MENTION=350383]crank_addict[/MENTION] to determine if you are getting lug interference. If that is the case, rotate the post with a flute located by the offending part of the lug and either try to extract it with back and forth rotations or GENTLY pry the edge away from the post enough to remove the interference. Usually it is a part of the lug that looks like a burr and can be eliminated if carefully done, no BFH should be applied!

Continue to add lubricant in the flutes to flush the junk out of the tube to reduce the resistance from the loose particles.

This is a trek 510/20? can't see the previous page. If so, the tube is straight gauge so you shouldn't have problems with the lower portion of the post corroded larger than the lug area. An option is to drive it in a 1/4" and then see if it will pull out easier. Just a dumb suggestion that might work!

Part of my solution was to apply heat to the post head. Not so much to melt the metal or burn the paint! Expansion with contraction can do wonders too. Use the Freeze Off to reduce the diameter of the post. I would not be in a hurray to pull out the hacksaw blade. You may want to let the PB Blaster soak during the move. My post soaked for weeks before I applied Kroil, letting it sit for a couple of days (week?) and then heat with full resignation that I would have to pull the blade out if I didn't get some kind of movement!