A few days of penetrating oil, then some heatings and coolings of that central bolt, will be helpful. Get the effer nice and hot. Then let it cool. That should break some of the internal corrosion free (yes it may melt a few rubber donuts, but that seal will be easily broken). I agree, you're probably best off buying a new bolt for reinstallation.
That extra nut freaks me out. When I removed my front ISO to install Mk III adjustables, I had a devil of a time getting it back in there, WITHOUT an extra nut. No way I could've fit that in there.
As a previous poster has suggested, you may have frame issues. Here's hoping NOT.
Best of luck.